A possible setup solution for IMS that I use may work for you also.

I am on T-Mobile for my primary voice line. In the account manager they have a section that controls roaming access and permissions. There are three choices for each line on the account. These controls are buried a few layers down in the Account heading, but will probably be different on your carrier. I need to access: Account-Settings-Permissions & Controls-International, to finally reach the relevant settings. Once I can see the controls, there are 3 choices: Allow All Roaming, Block Charged Roaming, Block International Roaming. I set my voice line to Block International Roaming and it will then always default to rejecting all International carriers. This prevents the handshake with any roaming carrier while overseas, and assures IMS is set properly to Use Cellular Data on that line. Moving regions, countries, or even continents does not require searching for a non roaming carrier. I even leave my voice line set this way at home since it should still be able to roam domestically.

I suggest a search or perhaps contact support for the possibility this type of configuration is available with your carrier. It may be a somewhat obscure setting, so you might have to really do some snooping. If this type of switch isn't a user controlled item, your carrier support can probably block all international roaming on your chosen voice line.

In years past I have had an occasional problem with areas having only one carrier. The Bahamas was one. BTC was the only service, so there was no non roaming carrier on T-Mobile, therefore IMS couldn't be set up! Having ALL international roaming disabled on one line is all that would work.