Welcome to Hubitat! Many people have got at least some Tuya devices working on Hubitat, but they are not officially supported--the workarounds you would need to do to make them work are partly a result of that. These devices are really only meant to work with the company's own cloud servers and other cloud services that they integrate directly with, like Alexa.
I know you already have the devices, but if you'd like to avoid this headache, the best way is to stick to Hubitat's list of compatible devices. These will all work with native drivers on Hubitat and little effort on your part (some LAN integrations may require a separate bridge/gateway, e.g., the Lutron switches, dimmers, and remotes on the list). Beyond this list, most Z-Wave and Zigbee (the primary protocols used by Hubitat) devices will work if they're similar enough to a supported device, but a search of the Community forum (or asking) can help verify.
I haven't personally used these devices, so many someone who has and has used the method you found (presumably this: [BETA]TuyaHubitat - (jinvoo, smart life, tuya smart - switches only)) can chime in. I can tell you that I did also find this other project: [Release] Tasmota Sonoff Hubitat Driver & Device Support. Safe to say, I've also never tried that, and I'm not sure if this (flashing the firmware to something else) would work on your specific devices, but it's worth reading to see.
That being said, it seems unlikely that such a small number of devices on your network--and presumably ones that don't communicate with heavy traffic--would be causing problems as your ISP suggests. Perhaps the wireless AP portion of what I assume is your modem/router/AP combo can only handle so many clients, and that could be causing problems; in such a case, neither of the above methods would help--the devices would still be communicating to Hubitat (or your local server) over Wi-Fi. Using a Z-Wave or Zigbee device instead would take those devices off your LAN and leave Hubitat as the one LAN-accessible interface between you and them. If that is truly the problem--and I'm still not convinced--then using such devices would be far more likely to help. Such devices are definitely what I'd look to going forward--if you add a lot, these protocols work much better for this purpose and won't add a bunch of clients to your Wi-Fi. Whether you think it's worth it at this point to switch (maybe you could sell them to recoup at least some of the cost?) or try to make what you work have work (with a bit of difficulty)--or investigate another platform or ditch the idea of Hubitat or a local hub entirely (probably wouldn't recommend the latter if you suspect you'll want a lot of devices)--is up to you.
