You can try plugging cable modem into WAN1 and ADSL into WAN2 for a comparison test. And did you configure load balance or link backup? Try disabling them.

@EIBROG What type of VPN did you configure? You can check if other users that can connect to ER605. If other users can and there is no change on router's setting, you may still focus on the single...

Not sure what you want to achieve, but usually this feature need to work together with a router. I think this YouTube video is useful: https://www.youtube.com/watch?v=7i17jvrIjD0

I saw on TP-Link website that EAP225-Wall only supports standard POE. It can't be power up by passive POE. I guess you may need to buy a POE switch....

Just pressing the reset button won't reset. You need to hold the button for 5 seconds until the light flashes orange then green. And normally, all TP-Link devices use admin/admin as default...