Hey, you may have a look at this: https://www.tp-link.com/us/faq-174.html

Can you detect the Wi-Fi signal and get internet with a smart phone at point B? If yes, you can choose one range extender to help you get the Wi-Fi and provide network at the same time....

Since you can only use wired connection, any tp-link router or switch will be fine. It depends on your the number of your clients.

How about if you insert the SIM card into your smart phone? Does it work properly? If the SIM card is working properly, refer to this FAQ for some troubleshooting....

Since you have also configured MR3420 as an Access Point, the DHCP function should be disabled on this router. Have you done that? If yes, it's unnecessarty to change the IP address of 3420 because...

Does that ip address you reserved use by other device? Try to change another different IP address to see whether you can set it successfully.