I am suffering from a similar problem now in Sweden. My Samsung Galaxy S8 functions correctly in the UK, but since arriving in Sweden yesterday my mobile roaming is not working. I have no access to calls, texts or mobile data, despite my phone showing full bars of signal. I have roaming switched on.
I have tried selecting network operators automatically, which doesn't do anything, and manually, which once resulted in a list of Swedish operators, but didn't allow me to join any of them. I have tried entering APN settings for Swedish operators, which hasn't worked, and have confirmed that my Virgin APN settings are correct (as expected, since my phone works in the UK).
I have tried restarting my phone multiple times, have removed and replaced the SIM card, and have checked for software updates. I never received a text welcoming me to another country, as has happened with other phones in the past, from Sweden or Belgium (on my stopover between flights).
Please let me know if there's anything else I can try.
[EDIT] I have tried swapping my SIM card with someone else's Swedish SIM for their S8, which worked correctly in my phone whereas mine didn't work in theirs. This implies it is a problem with my SIM card rather than a hardware issue.
Last year my problem turned out to be that Roaming was not turned on in the account even though online it said it was. A kind person from Virgin turned it on and all was OK. That my fit in with that your issue is SIM related.
The matter actually seems to have resolved itself. After trying a bunch of things we had basically given up, then late in the afternoon all the messages suddenly came through. I imagine that it was a case of the roaming settings not catching up with the phone settings until then. Weird that it took so long. Thanks for telling me about that app, I can see the settings so I'll be able to try that if this happens again in the future.