First creating a IOT network is simply putting your network into a compatibility mode basically. From a Unifi perspective they simply adjust the settings used for that SSID to improve compatibility. It shouldn't be a requirement for something to work though. I have serious concerns if they are asking you to do this so you can turn on WPA security. That is a problem with their device and firmware. WPA is extremely old at this point and extremely insecure. It is just barely better then WEP at this point. Everything should aim for WPA3 support if you want security of your network. Can you set everything back the way it was before you started to try to fix this issue and see if that gets everything functioning except the Rheem device.
What I am suspecting is that you may be having a issue with the radio signal at that device for some reason. Wifi is great when if it just works but because it is wireless tech all kind of shenanigans can cause poor or weak signal for a device which can wreak havoc on how it works. It may have nothing to do with your network and simply the antenna in that device is weak. You could have a Mesh AP that has a failing radio causing issues and it could of been masked by other AP devices. I would ask you go to exactly where that Rheem device is and check the radio signal with something like wifiman on your phone.
Now as far as Unifi goes there are a few things that you need to understand when considering it. First as it was eluded to by @johnwill1 Unifi isn't exactly as simple as just going and buying a router. Unifi has a Network App that acts as a controller for all other devices on the network. Some devices include the ability to run the Unifi Network App while others do not. For home use I personally wouldn't consider a devices that doesn't include the ability to run the Unifi apps on the router. Also because some of those same devices are also designed to be good options for SOHO/Small office setup some do not include a AP in the Router/Controler device. So you need to consider that when deciding on which one to get and buy AP's accordingly.
The basic breakdown is all of the Unifi Cloud Gateway devices are router gateway only so you would need to buy a AP to go with them. Be careful because they do have devices labeled "Unifi Gateway" that don't include the ability to run the network app and manage other devices.
The Unifi Express line of devices are intended to be the most basic options, but include a AP in them. These are the simplest option, but generally the least feature rich. The nice thing about these devices is that they can also function as a AP if you outgrow them and decide to implement another device to control your network. At this point don't get older one as it is under powered and a waist.
Then you have the Dream Routers. These are interesting devices because they have bit more functionality then the express devices, but still lack some of the functionality of the UCG comparable devices. They include an AP and tend to be bigger then the comparable devices of that generation with similar functions.
That said the UX7 and the UDR7 were just released so they are very compelling compared to the other stuff.
Lastly keep in mind most of the Unifi Gear will require you to use POE to power them. Some of the gear has a POE port or two while some have non. If you don't get a device that can supply POE you will need to buy POE Injectors to provide the needed power. I ended up with a Flex 2.5 8 POE switch to supply power for everything.
I have a UCG Max, 1xU6+ AP, and 2 U7 Pro Wall AP's which for the most part has been a positive experience. If I was to buy it today I would probably pick up a UCG-Fiber instead of the max though as it has 10Gbe that can be used for some internal network to speed stuff up. Seriously though that would be overkill. I might skip the U6+, and get the U7-Lite, but i am not sure. The U7 Gear has had some teething issues.
I upgraded from a Google Wifi Gen1 kit with 3 pucks. It used Wifi 5 AC Wave 1 tech and was reaching it's capacity of network size. It seems to struggle once you get around 80-90 (25-30 devices per AP) devices. This is pretty much typical with any Wifi network of that gen or older. I have actually experienced some weird Wifi issues myself as of late. The cause though isn't very straight forward. My problem is I have a few devices that clearly don't have the best antenna's, and also support 5/6ghz radio for faster communication. Because the majority of my network is 2.4 ghz IoT type devices any time the problem devices would fall back to the 2.4Ghz band, network performance was abysmal for them. I tried all kinds of tweaks like adjusting the positions of the AP's as well as adjusting transmit power settings for the various bands. I am still tweaking this trying to figure out why the connectivity is spotty for those couple of devices. My next step will be to simply add a 4th AP as it appears to be a coverage issue of the 5/6ghz network. One of the last things I did was change how i mounted the U6+ to be similar to a ceiling mount like it is designed for. That should help that AP work better and we will see if that does it. Setting up good Wifi is damn near a artform when you consider some of these nuances.
Why I would need a 4th ap with Unifi when I functioned fine with the 3 pucks with google wifi I have no idea. My WAF has taking a hit with this change as it is her devices that seem to be impacted. I can sit in the room and get near gigabit speeds on my phone while her tablet drops down to 2.4 and can barely stream youtube.
After all that being said it should work fine and would be very easy to create a IOT network no matter what gear you get. I am not convinced though that there isn't something else causing the problem you are dealing with. It could fix it, but it may not fix anything just as easily, Be prepared to tweak the Unifi network as well.
If you do it considering you have the TP-Link ER605 the most similar setup would likely be something like a UCG-Ultra, UCG-Max, or UCG Fiber and then get either the U7 Pro XG, U7 Pro Wall AP's if you want 6Ghz, or the U7 Lite, U7 In-wall if you don't care about the 6Ghz band. Don't forget something to power devices over POE so either Injectors or a switch like Flex 2.5g 8 PoE
with wifi isolation on
