My recent experience when roaming was that calls only worked on 2G even though 4G and 5G were avaliable. After a call, phone would sometimes stay stuck on 2G which meant internet was not working. I had to disable 2G to force the phone to switch to 5G. But if I forgot to enable 2G again, calls didn't work. I checked this with multiple people and they all had the same issue.
Then your carrier has a shitty roaming agreement where you were roaming. Enabling the 4G and 5G bearers is an option they can choose to enable (and pay for) their customers — or not.