Just had a pointless chat with 48 support due to a coverage issue my wife and son were having after flying to Germany. They were giving me APN settings to change to fix network coverage which is not correct at all. Network and data are two diferent things

I got it to work for them myself and here are the steps we used.

What is going on with your 48 SIM when roaming:

If you are taking your 48 SIM abroad, you may get no network coverage when first turned on in the foreign location.

These are MVNO SIMs and for roaming they use a dual-IMSI SIM to literally have you acting as one of two different mobile subscribers within the one SIM card. They do this to get cheaper roaming rates for both calls and data. They can't charge you for those costs in the EU, so they have to get the cheapest deal for them directly. Hence the SIM is configured so it can operate as a Belgacom-Proximus subscriber when you are roaming.

When you roam, you possibly need to manually switch that identity depending on where you are roaming to.

Switching MVNO IMSI Identity

On your phone there should be an app called "SIM Toolkit" or something in that ballpark. It may also appear as a settings option in your SIM/Mobile network section (the same place where roaming can be turned on/off). I'm speaking from an Android perspective here but something similar should apply for iPhones.

Launch that App and navigate through the menus as follows using either approach. Try the other if the first makes no difference. It lists all kinds of options but at the bottom there should be an option called "When Abroad". 

Then select the options as follows:

When Abroad -> Network Selection -> Automatic  (usually the best one to use)

or.. 

When Abroad -> Network Selection -> Manual -> International Roaming Profile 

When you bo back on the SIM Toolkit menus and navigate forward again, you should see an asterix (*) alongside the selected option confirming it has been changed to what you requested.

Then when you exit back to the home screen, you should hopefully start seeing the phone getting a network connection... and by that we mean coverage on some carrier and not the emergency calls only. The network coverage symbol should not show an "X" or "?". You should see coverage bars etc.

If that is not working, put the phone in airplane mode for about 20 seconds and then turn that mode off again to see if it can now attach to a roaming network. 

If that will not work, try alternating the SIM Toolkit menus again between automatic and manual as shown above and try again. 

Once you get that coverage sorted, the phone should be able to make and receive calls. Go no further until you can get that working.

APN Settings for Internet

Then you can check data services (Internet etc). For data, on the settings -> SIM section, make sure the data roaming is turned on and then check the APNs or Access Point Names.

So when you get into that APNs section, select the 48 profile if present and exit out and test internet (with wifi turned off).

If you see no APNs or just Belgacom/Proximus profiles, then use the "+" option to add a new profile. Set the name to 48 and APN to 48months.liffeytelecom.com. Leave all other options as-is. then on the top-right dot menu select the save option. When you get back to the APNs selection screen, select the newly created 48 profile to be sure its the one that is active. you can also edit the existing 48 profile to confirm that the APN field is set to 48months.liffeytelecom.com if need be.

then exit out of all menus etc, make sure WiFi is off and launch a browser and say go to rte.ie or something like that to confirm the phone is getting data services.