@Steffffi Here is a config for a similar solution. the router is connected to the LAN with only the WAN port you get to test and play a bit and you'll probably figure it out :-)

@Steffffi You probably have port NAT on fortigate :-) you NAT the wireguard port on the first ER605 to the second ER605 WAN interface so that you can reach it from the internet.

@Steffffi because you are routing back to the LAN with this configuration. AllowedIPs = 0.0.0.0/1, 128.0.0.0/1 or AllowedIPs = 0.0.0.0/0 so you have to connect like this internett ---- WAN first...

@Steffffi no, the tp-link routers are not switches, but as I see it, you only need to connect to the WAN on the ER605. I have some similar solutions with other manufacturers and there I have only...

@Steffffi what does your config file look like? try Allowed ip AllowedIPs = 0.0.0.0/1, 128.0.0.0/1 or AllowedIPs = 0.0.0.0/0

@Almacratos no problem