Data bolt on not working properly

[Thals]

I'm not sure if this is the right place but for some reason my data bolt on isn't working properly. Before i ran out of data my 4G worked fine but now that i've switched to the add on, if i'm not connected to the wifi, my phone has extremely poor internet connection, and sometimes it does not work at all, i'm just wondering if anyone knows why or how to help?  
This isn't the only time this has happened either, when i used a data bolt on a few months ago, this also happened and i couldnt access the internet! (I was also in a different area when this happened before so i dont think its a signal/mast problem)

[MI5]

@Thals 

Have you rebooted the phone?

[jonsie]

What phone do you have @Thals 

[Thals]

Sorry, ive got an iPhone XR and yeah ive tried rebooting, it didnt change anything!

[jonsie]

Maybe the data bolt on has changed. You need to call them or change the APN to mobile.o2.co.uk

[Thals]

Changed how? and whats an APN?

[MI5]

The data apn is in network settings and should normally be set to iData.o2.co.uk.
Using the alternative may work, usually when abroad, but specific iPhone features won't be enabled.
It's strange that it works until you try and use a bolt on but that shouldn't make any difference at all.