Statically assigned DHCP issue on carrier modem

I've been having an issue with a carrier modem and a meraki mx64.

Essentially we have the mx64 set up for a private 192.168 network, and a statically assigned gateway of 1.1.

We generally request the carrier set up their modem for static ips for outside the network and private ips inside the network, statically assigning the modem to be 192.168.1.1 for the gateway ip.

But I've had a couple of sites now where I see the carrier modem advertising as the 192.168 ip on the switchport, and after a little while it changes to the carrier public static gateway ip, then back and forth. About twice an hour.

Side note, the carrier static IP that actually gets assigned to the mx64 is way outside what's actually in the subnet. A .212 when the gateway is a .6 /30.

What's going on here? I feel like it's related to dhcp somehow.

(I know the easiest fix is just get more static ips, and set the mx64 up for a nice tite static block, but the customer dunnae want to. So cant. And convincing otherwise is a no go. Other sites work this way. U_u)