May I know what firewall do you use? Based on your experience, next time when i meet the same issue, i will first try as what you've done.

The channel could cause such a problem. The available wireless channel varies from countries. The possible solution is to make your wifi signal running on a specific Channel within 1 to 11.

Check the default settings on the label attached on the router itself. Is it 192.168.0.254 or 192.168.1.253? Where did you buy it?

It seems that your ZTE MF825A is not on the router's compatibility list which can be seen here http://www.tp-link.com/en/support/3g-comp-list/?model=TL-MR3020&location=1151. These two might are...

I don't find the ZTE MF820D+ listed on the router's compatibility list: http://www.tp-link.com/en/support/3g-comp-list/?model=TL-MR3420&location=1144. TPLINK possibly have not tested their router...

Try admin for both the login username and password.