Replacing ISP Router Issues

My modem doesn't provide ethernet unless directly plugged into the router from my ISP. I have my OpenWRT router/network connected to that ISP router. My ISP said they don't think the device locked by mac address but I don't trust them at all.

A friend told me, getting a static IP was the work around. When I check my ISP router, I can see this IP, a special subnet, PPPoE and DHCP is off. I've mimiced this by changing my WAN to Static IP, and using this info. Packets move instantly then throttle.

Every other thread related gave short answers but no one has broken down how to troubleshot at this stage. So far I've seen people talking about packet tagging, wire shark, and vlans.

Please advise the answer and how to systematically troubleshoot this issue. Thanks.

Nonetheless, have you tried setting the WAN MAC of the OpenWrt to match the WAN MAC of the ISP device you wanna swap/replace?