Does your switch connect directly to gateway or are there any other managable switches? If directly, then as Andone said, configure uplink port (port which goes to gateway) as vlan 30/40 tagged. If...

VPN will use only 1 WAN, that's true, because it is a point-to-point tunnel. Moreover, VPN will lower your speed, because of encryption. Usually, you can see maximum throughput on specificatios....

Check supported 5Ghz channels on your devices. Configure 5Ghz channel manually on your AP. Usually, it is because end device just doesn't support Band1 (channels 36-44) or Band2 or Band3.

TP-Link doesn't recommend to have more than 15 clients, if you do not use TDMA, which you will not use on mobile phones.

Don't mess the subnetworks, that's true :) First of all, why do you use WISP, if you have router after CPE. Are you making one more NAT? I would configure it just AP<->Client. WAN940<->LAN940...

If you are talking about projects, then normally you should have special software, which can calculate the coverage based on floor plan. Also, you can do active coverage research with such software...