@jnydep 

Can you describe exactly what you're seeing in the browser, when it fails to connect to the router's management page (including the error message, how long it takes to time-out, does your browser "automatically" change the URL/address you typed into something else for example: tplinkwifi.net, etc.).

Also, in the browser's address bar, what are you typing to connect to your router: "http://..." or "https://..."?

Does your browser have anything (an extension or in its setup) that may force it to use HTTPS? (this would cause problems if your C7 router is setup to only use HTTP).

Since your network has to have a router (not your C7 since it isn't setup as a router, but another device acting as the router), you should go to that router's management page and look at the devices connected. In the list of connected devices, look for the IP address assigned to your Archer C7. That's the real IP address to use, when you try to access the C7.

Lastly, you may have tried this but do this if you haven't: (1) remove the C7 from everything (remove all communications/ethernet cables). (2) plug an ethernet cable into the C7's rear "Ethernet" port (any one) and the other end of that cable to a computer. (3) On that computer, disable its WiFi (temporarily). During this, we don't want it to connect to the C7 using WiFi. (4) On the computer, use your browser to access the C7 (it should use the wired ethernet to connect). (5) After you're done, on the computer re-enable its WiFi.

If the above doesn't work, repeat it but in step 2, plug the ethernet cable into the C7's "Internet" port. Finish repeating the steps, and write down what you see happens.

Based on what info you reply with to my questions, it'll tell us what's going on.

FYI: Your post at the top says your device's model is "TL-WR850N" (https://service-provider.tp-link.com/wifi-router/tl-wr850n/#overview), and not a C7. I don't believe that device is the same as an Archer C7 (https://www.tp-link.com/us/home-networking/wifi-router/archer-c7/).

  @jnydep 

There's an identical post from a few years ago, that you can read. It has suggestions that worked for others.

That earlier post:  Can't Access Router's Management Page After WDS Bridge Setup