Do your mean powerline light is off? You got two? PA4010 and WPA4220? Run the utility for PA4010 and reset both of them. After reset, check if powerline light will be on. Or try to set the same...

MR3020 works in 3G/4G router mode? Internet access is 3G/4G Only? Are you connecting a PC to its LAN port? Check status of local area connection on your PC. Any IP address, default gateway etc....

An access point will have to connect to the root router by cable connection? Is it what you need? To extend Wi-Fi range and connect to the root router wirelessly, it will be repeater or range...

So now the Wi-Fi disconnection issue is fixed? On their website, it notes, TL-WR720N V2 contains one external fixed antenna, while TL-WR720 V1 contains one internal antenna. I don't know the...

I doubt the USB port on MR3220 is broken. On web interface of MR3220, under 3G/4G, is it showing unplugged? When did you buy it? Maybe it is still under warranty?

I read that thread but I didn't realized that it would work in your case. So far so good?