1. If you don't need the router part of the ISP's modem/router, you should enable "bridge mode" to make it behave like a regular modem. Alternately, you could configure your router to behave as an access point. That means DHCP LAN IP addresses still come from the ISP's modem/router and it's one big LAN for all devices, regardless of which access point your devices are connected to. If your router doesn't have an AP mode, you can disable the DHCP server and connect the incoming connection to one of its LAN ports to have the same effect.
2. If you have to do double-NAT (router behind a router), check to see if the ISP's modem/router has a DMZ option. You can point all incoming connections to your router. In your router, you configure whichever ports you want to forward to your PC behind it.
If you have to do this, see if you can reserve LAN IPs in the ISP's modem/router and reserve one for your router. If the IP on the WAN interface of your router changes when equipment reboots, your forwarding rules will be broken. If there's no reservation feature, adjust the DHCP range so .100 is the first address, then manually assign .99 as the WAN IP address in the WAN/Internet configuration of your own router. That way, it won't change and it won't conflict with any DHCP addresses your ISP's modem/router gives to another device.
In your router, you'll also want to configure a DHCP reservation for the PC/server's LAN IP (or configure a static IP outside your router's DHCP range on the server machine, if you know how).
What model is your ISP's modem?