The only thing which may cause the issue you have is the TL-WR841N. You can go ahead and set up the TL-WR841N as an Access point and leave the others the same. You may refer to this link to set up...

Here is the new firmware: http://www.tp-link.com/en/download/TL-WPA4220KIT_V2.html#Firmware If you want to upgrade, you need to download it from the website and do it manually on a computer. I won't...

You can get the info here: http://www.tp-link.com/en/faq-434.html

The beta firmware is in this link: http://static.tp-link.com/MR6400_voip_beta_2016-09-06_15.01.35_1474602142560j.zip

This may be related to DNS and MTU settings. You may go to the management page of MR3420 and go to Network-->3G/4G-->Advanced to change MTU to a smaller size, and go to DHCP settings to change DNS to...

I think it may not work. As on the spec of TP-Link website: http://uk.tp-link.com/products/details/cat-5258_HS100.html#specifications, the maximum load for HS100 is 2.99KW.