2 routers connected in series and port forwarding

Unless you are able to get into the router of your ISP or replace it by a different router, there are no options.

The traffic goes as follows: Internet > ISP router > Your router > Your server.

In order for a package to be able to fully reach your server, the ports have to be forwarded on both the ISP Router and Your router.

It may be possible to ask your ISP to set the router in bridged mode. It will assume a router is attached to the modem/router and it will forward all ports automatically to the first LAN port.

EDIT: There is one more thing you can try. Google for your ISP's name, plus router password. They often set a standard password to all their devices, and may have told someone in the past who put it on the internet.