The current firmware version of WR702N showing on Status page is 141203? If so, upgrade should succeed.

Do you have to change any settings on Asterisk server after you use a new router? The router's WAN is public or private? Maybe you have to open some ports for the server like UDP 5061 and 5060?

What's the WAN IP address obtained on web interface of 8840T when access internet by the modem directly?

With new UI, it is not called firewall anymore. Some of functions under firewall can be fulfilled in different sections on the new UI. What kinds of feaure do you need?

Did you try to plug any USB driver to see if it works on that PC? Ask some computer technician to check your PC.

Are you using Windows built-in utility or TP-LINK utility to connect Wi-Fi? With either of them, there is an option to connect automatically. It won't work? In profile list, it is not saved there?...