For the wifi, I guess there are many interference around. So if you can, connect the TV to the AV600 via Ethernet cable. Also check the link rate with the utility as Peter suggested. BTW, what's the...

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...