@AJC01 ok, then you need to fix that before you can set up site to site vpn. you can't have a private ip on the router. Have you tried removing the ISP router and connecting the ER605 directly to the...

@AJC01 ok, then it won't work, you have to talk to your internet provider about putting the modem in bridge mode or getting a public ip Do you have private IPs on both routers or is it just one?

AJC01 wrote at the wan-ports, i have the lan adress of the isp modem (=gateway) @AJC01 it won't work, what ip is it? does it start with 192.168.x.x or 10.x.x.x

@AJC01 no no ports should be opened. when it works you should be able to ping gateway ip from lan to lan, if you get a response then vpn is established

@AJC01 Do you have a public IP on the WAN interface of the router? You must have that on both routers or it won't work.

@AJC01 first you should get the vpn up and running then you can concentrate on ports then. but to block and open ports you use acl which is also described in the manual i would think