-On your ISP supplied modem, set it to “Transparent Bridging” this will stop double NAT and is the best option.
-You ISPs modem will connect to your ISP via its WAN port. You then need to put a cable between a LAN port of the ISP modem and a WAN port on your Peplink.
-DHCP should work for the WAN settings on the Pep. If it doesn’t, you might need to use static or even PPOE by comping the settings which are in your ISP router (MAC address spoofing could also be needed)
Any questions, just ask!