Repeater mode is not recommended in long distance communication. It's better for you to setup two 7210 to build a P2P link as below: In this case, you have to use a extra AP to cover the indoor.

What's the firmware version of the device? It's unusual that no test mode on the resion page. To adjust the transmit power, you can log in the web management page and navigate to WIRELESS -> Wireless...

Log in the web management page of C20i, navigate to wireless 2.4GHz/5GHz > Wireless Advanced to check if the Enable Client Isolation is selected. If yes, cancle the selection.

Log in the web management page of C5, navigate to Advanced -> Forwarding -> Virtual Servers. Add a new item: Service port: 8080(any number between 1-65535 you like, 8080 for example) - Internal...

I think #2 has the correct direction. But you can have a test to connect to 5110G in front of it to eliminate the possiblity of distance. 5110G is a outdoor AP that it broadcast strong signal....

@danymarc Very helpful information, thank you. But 7210N is a outdoor product. It performs well in outdoor long distance scenarios. If placing it indoor as a wireless router, anyway I think it's a...