So the bottom line is that if you need full admin control of the hub your PC has to be on a subnet that the hub can see and respond to. Most direct would be to place dual NICs in the PC and connect to both routers (can do the same with a WiFi connection to one and physical to the other). Alternatively you can setup a VPN on the hub router and tunnel back in. Could also look at connecting the 2 routers and adding the additional subnet to the hub via the endpoint http://<Hub IP Address>/hub/allowSubnets?<Second Subnet, i.e. 123.123.123.0>,<Third Subnet>,…, or place one of the routers in AP mode (make it a slave of the first).
Others can probably comment on this more accurately than me.... But the optional Remote Admin subscription provides you with the ability to access the admin features of your hub remotely without the need to configure something like a VPN to provide your access, which some users would not feel comfortable configuring or maintaining.
Can you describe in more detail how your network subnets are configured?
What do you mean that router A is a “repeater?” As in a WiFi repeater/extender?
I could be misunderstanding, but your scenario doesn’t sound like one that remote admin is intended to address. It’s essentially a zero-configuration VPN that allows Hubitat users to connect to their hub’s full admin interface while they are away from their LAN. It runs through Hubitat’s cloud, and it is offered as a convenience for users that would prefer not to setup their own VPN server.
With that in mind, why wouldn’t they charge users for that optional service?
You have to be on the same subnet or create vlan's and give access to those vlans to both hubitat and your pc. Right now you're basically internal double natting it sounds like and you won't be able to properly talk to the the subnet that the hubitat is on if you aren't on that same subnet. Connect router A directly to the internet via it's wan port. Connect router b via it's normal switch ports to the switch ports on router A. Disable DHCP in router B and do not use router b's WAN port. This will flatten your network and you'll be able to talk to things properly.