Both my wife and I had similar problems in Spain and Portugal in the last fortnight. Everything was fine the day we arrived in Bilbao but sometime the first day as we drove south mobile data was lost. Tried everything with all the settings. It was quite serious as we were out in the sticks in Spain, had no navigation, no way to access covid info for entering Portugal, all sorts of things you take for granted. Had an "interesting" drive around the outskirts of Seville too.
Got on to wifi on a tablet when in Portugal and eventually got it working after a long chat session to support. They initially tried to fob me off but when I pointed out that I am going to report the issue to Comreg the tone changed and I was talked through APN settings, all of which I had already tried but somehow after I logged off chat and then toggled all my apps' data access (again) full data roaming was restored. It dropped again in the north of spain at the end of the trip but sacrificing a goat, crossing our fingers and waving our phones at the crescent moon seemed to sort it out again.
I am going to make a general point to Comreg but I can't be bothered with all the negativity of getting into a fight with Three/48 but seeing the numerous complaints on these forums it's obvious they have had an issue for quite some time and haven't being taking it seriously.
I'm not leaving 48, they promised a service and I'm expecting them to provide it. Persevere and talk to them about talking to Comreg and get them to take this seriously before someone gets in a real tricky situation abroad due to loss of data.
If the links in this post don't help then you'll have to contact 48 support.
Unfortunately lots of people have been having problems with roaming lately and some never get it working at all.
Just back from Lisbon. Both my wife and I use 48 as network provider - she had no problem and had data. I am techi so tried everything and could not get data connection, Was able to connect to a network but still no data. Eventually swapped sims and got data on my phone and no data on wifes phone. conclusion 48 sim problem not phone