If the laptop can not connect , how about other devices? How about other channels?If you leave the channel width as automatic, how is the things going?

When there is no Internet Access, can the computer still get IP address for the wireless adapter from the wireless router? If the computer can get the IP, how about the signal form the router? What...

Make sure that the 3020 is switch to 3G mode, then connect the 3020 by an Ethenet cable. Try 192.168.0.254.

You can change the MTU to 1440 to give a try. 1492 is the most service providers mtu size. Change it to smaller than the one they provided. And for the wireless, you can also change the channel wdth...

Maybe you can give a try on Parental control for wireless user B. Control the time and the website that he /she can access however there is no restricts for LAN. Not quite sure whether this can fit...

Check the lights on the router. 5G wifi LED light should be flashing. I do not think your laptop wireless card is compatible with 5G band. Another way is find another device that can work on 5g band...