The problem sounds like your network is being divided into 2 subnets, even though you attempted to follow the setup instructions to make it a single network. One thing you could try is to delete ALL computers from the Win7 Homegroup, and recreate the Homegroup on whichever downstairs computer you originally designated as the Homegroup master. Rejoin each Win7 or Win10 computer to the Homegroup with the newly created Homegroup password. Make sure that all computers are set to the identical
WORKGROUP name as well, and that none of those computers are inadvertently set to log in to a Domain controller. (Doubtful you have a server in your LAN, but this can cause problems with that wrong setting).
If none of this works; chances are you are having subnetting issues, and your 2 routers are not capable of doing this correctly, which we often see with TP link devices as their firmware and drivers are terrible IMO.
Personally, whenever I encounter home networks with any TP equipment I immediately rip them out and replace with name brand cisco/Linksys or netgear only.
That may offend you if you've had good success with TP devices, but I do this for a living and they make my life miserable whenever I encounter them. Others may have different experiences.
The other thing I believe you are doing wrong is using a 2nd identical router for the upstairs, as you should be using a Range Extender for that purpose, not a 2nd router. The 2nd router not being connected to the downstream port of your 1st router (WAN port) can be a problem, again TP does not do a good job of firmware support for multiple-router single network inclusion. In fact, I've never encountered a setup such as you have in the field that actually works with all connected computers. I would rip out the Homegroup and recreate that on your LAN on all computers. If that fails, I'd replace both routers with 1 name brand router and 1 name brand Range Extender (type N or AC), and you will see those problems disappear!
Best of luck,
<<<BIGBEARJEDI>>>