If it’s double NAT where you have control over both boxes, it’s not that big a deal. First of all, it only matters at all if you’re trying to forward ports for remote access to your services, in which case you just need to add two port forwarding rules for each service, instead of one, one in each firewall. Alternatively if the ISP router allows it, see if it has a 1:1 NAT feature, this way it forwards ALL the ports to your private router, where you can then be selective about which ports to allow.
Alternatively, if you’re not trying to host services on your LAN for public access and consumption (Which would be a bad idea at this point in time anyway given your level of knowledge) don’t worry about the NAT or port forwarding at all and just use a mesh VPN like Tailscale (Optionally with the self hosted control application Headscale) and use that to access your services which outside home securely.