@terziyski 

I already tried that before and was still having the same problem, I can try to switch the port again now but am skeptical of the outcome. Also ever since I set the static IP I can't access the router's configuration interface, neither at the routers static IP, nor at tplinkap.net.

EDIT: I switched the cable to the WAN port and that made no difference, the WAN light on the router is orange.

@terziyski 

What should the default gateway and DNS IPs be set to, the same as the static IP, the IP of the main router or something else? I can't leave them blank on the AP router.

  @aboninski 

If the switch modem is bridge mode then router B (and router A) must be router mode.

Another way is connect AP router B to Main router A.

  @rane61 

To be fair, I'm not sure what mode the switch is running in, its one of those plug-and-play tplink small boxes.

  @terziyski 

The topology can be seen in the picture of the first post. My main router's LAN IP address is 192.168.0.1 and it's DHCP IP pool is 192.168.0.2-192.168.0.254 with a 255.255.255.0 netmask. On the other hand, when I open the access point router's configuration for the first time after setting it in AP mode I see that its LAN address is entirely different - 10.108.183.13 and its default gateway is 10.108.183.1, that's per the initial automatic configuration. The gateway, configured in the main router is the same - 10.108.183.1. When I configure the AP router with a static IP - 192.168.0.254 - and set its gateway and NAT servers to 192.168.0.254 (LAN IP of the main router), I no longer can access AP router's configuration interface. It however manages to provide internet access to some devices connected to it and those of them that have internet access have an IP like 10.108.xxx.xxx. The AP router however issues IP's seemingly from the main router's DHCP pool - 192.168.xxx.xxx but devices with those IPs have no internet access.

Here are screenshots of my main router's configuration:

I'd expect if the configuration is correct and everyting starts working properly, that devices will only claim IP addresses from the main router's DHCP pool and not from some random one.

  @terziyski 

if I set them both in AP mode should I disable DHCP on both of them or neither? Also do I assign static IPs to them?