This isn't the answer you're looking for, but I recommend that you not use a router given to you by your ISP. Your router is also your firewall, and do you want your ISP controlling the software and settings on your firewall?
The answer you're looking for is that yes, a VPN could help. You could set up an OpenVPN server on AWS or google, do the port-forwarding there, and use the AWS OpenVPN server's IP address for external access. However, that begs the question: Why not just put that webserver on AWS? Maybe there is a reason (e.g., if the webserver is your NextCloud home cloud server).
Also, if your router is configured for UPnP, you may be able to use that to automatically forward ports. You might have to install an mDNS server on your webserver machine (e.g., bonjour, zeroconf, rendezvous) to announce the webserver to the UPnP service on your router.
Honestly, I'd contact the ISP, I'd get a modem supported by the ISP, buy my own router, and configure it for port-forwarding. Short of that, I'd call the ISP to give me access to the firewall rules and add port forwarding.