@Kurogane What is the current firmware version of your router? Please ensure that you have connected your smart phone to the TL-WR841N's wifi; Try to change another phone or reinstall the Tether app...

@fnam What about the stability of 2.4GHz wireless connection？ Try to change another PCI-e slot to plug it and move it closer to the main router. Besides, you can try to modify the wireless settings...

@zamogonz Definitely sure; if need more help, please be free to tell us. Best regards.

@Reddy How do you connect them together? What is the operation mode of the TL-MR3020 you want to use? Cause it is a hotspot, it should be working already and you can get internet from it; do you want...

@Nirobi This is TP-Link community, we are not familiar with these devices. While TP-Link do have some 3/4G routers. https://www.tp-link.com/en/home-networking/3g-4g-router/ You can pick one of them...

@bpascua You can try to swap the locations among the three access points. And you can reserve an IP for the 801ND V3 on the web UI of the main router or disable the DHCP server and assign it a static...