I'm a new member, and had a weekend in the UK where I couldn't get online at all. I was able to receive texts (tried using the tweaker site that sent me an SMS).
I was very disappointed with the service of course but I have to say the support people were great. Over wifi I text-talked to two people on the website chat who offered advice about APN settings, and then I started messaging the Facebook page where I got reasonably timely responses.
For future reference:
My phone (S7edge) only ever connected to the O2 UK network. I changed the APN to 48months.liffeytelecom.com etc, but no luck.
If I changed the MCC to 206 and MNC to 01 (as suggested by one support person) my phone refused to let me see or edit the APN settings at all.
I changed the settings multiple times. I rebooted my phone, I toggled data on/off, I made sure data roaming was on. However I wanted to enjoy my holiday and I didn't want to spend all my time fiddling with settings so I gave up in the end. Typing support queries on a phone screen, then switching to the settings screen to test got frustrating.
My wife complained that she couldn't ring me, but I may have reset the APN settings at that time out of frustration so didn't test that properly. (SMSes did arrive from the tweak site earlier while I was attempting to fix things)
One final suggestion from support was to try the SIM in another phone but we were so tired of trying things without success I didn't bother. My phone used to work fine on Meteor/Eir roaming. Could it be the SIM itself?
I write this wondering if I'm not alone in resolving roaming problems at all?
I'm back in Ireland now so I can't test anything else, sorry.