How to get Peplink Balance One core router to forget my old IP address?

The title sums it up - I’m trying to have Peplink router forget my IP address, how to do this?

I tried to get a new IP address on my Peplink Balance One core router, but the only way I could do this for the cable modem part was to switch the WAN ports.

I have a cable modem and DSL going in through the router. If I unplug the DSL and keep it unplugged / disconnected, then reconnect it back through the Peplink (same WAN port) there is a new public IP address in the dashboard by the DSL’s WAN.

If I connect the cable modem directly to the PC, it gives me a different public IP address. If I unplug/power down the cable modem, and then connect it again directly to the PC, it gives me a new different IP address.

BUT…if I connect the cable modem to the Peplink, it gives me one IP address. If I disconnect / power everything down, and leave it unplugged for 10+ hours, and put it back in, the Peplink gives the same public IP address.

Because the cable modem direct to PC can produce different IP addresses, I think the solution is in my Peplink router. However, I can’t figure out what could have caused this. Are there any settings I can check so that it doesn’t remember my IP address in the future?

Thanks for any pointers.