mrbrunes :
Hi
All the guides about setting up an old router as an wifi AP recommend using the LAN port for the router's LAN connection and not connecting the WAN port. The issue is that by doing this that the router itself can't connect to the internet as it uses the the internet/wan port for this.
Anyone know how to solve this?
No, incorrect. If you use the WAN port you will create a second subnet, which is okay if properly configured and you want privacy from the rest of your network.
To use a router as an AP: attach from main router to AP router LAN to LAN with a cable or pair of PL adapters, turn DHCP off on the AP router (that is all handled by the main router), set address of AP router as an address in the network range but outside the main router DHCP range (to avoid conflicts), give the AP router a gateway IP and DNS address equal to that of the main router (so like 192.168.0.1 or whatever).
You can use the three free LAN ports on the AP router for wired connection as they simply act as a switch and will not interfere with the wireless AP function.
Insure that the AP router has a different set of wireless channels used than the main router to avoid interference. You can use the same or different SSID and passkey -- I use different SSID because client devices are stupid (lacking 802.11 r, k, v) which the router won't have.
The AP will be invisible to devices and the main router will do all the routing work. Very simple and always works.