You can try as attached picture.

Where's your location? You can contact local technical support about this issue. I think they will help you change a power adapter.

Usually in system tools--Administration.

Do you think it's related with MTU settings? If yes,I think tplink should add MTU settings in the management page. Maybe change to lower MTU could solve the problem. Beside,is other SIM card have the...

When signal is weak,the device(3020) need more power to receive signal. So I think maybe the power supply is not enough on the bus? The standard input of 3020 is 5V/1A.

I am not familiar with antenna. But I think maybe and SMA extension cord will work for your case. Connect the extension cord to MR6400 and put the original antenna where signal is strong.