Lan Wan conflict resolution...again

I’ve seen similar questions in older threads but I didn’t want to bump one from two years ago so here goes.
I have a Flexy in the field with a LAN ip of 192.168.100.2. There are several other devices connected to the Flexy via a static ip address on the same subnet with a /24 mask. I don’t want to change any of these ip addresses or the LAN ip address of my Flexy. I have the Flexy WAN set up to receive an ip address via DHCP, but the customer has been assigned a 192.168.100.XXX static ip address by their provider and they say they can’t change it. As a result, the Flexy won’t connect to eCatcher and I can’t remote into the machine.
Can we have them put a router on their network so it will assign a DHCP address on a different subnet to my Flexy WAN? How would you suggest we get this connected if I don’t want to change all of my LAN ip addresses to a different subnet and they can’t/won’t change their ip address so the WAN is on a different subnet?

Yes, you can put a router on the WAN side so that you do not have an subnet conflict.