Openreach should update it, regardless of ISP.
If the modem is locked you won't be able to access it so it doesn't matter what its IP address is, it won't clash as it's on the WAN side of your router.
If it's unlocked then you can access the stats over the one wire by giving the modem a private address outside of your LAN and unticking the bound LAN1 port. Because the address is outside your LAN, your router will forward the packets down the default gateway.
Edited by deleted (Thu 07-Jul-16 19:56:54)