iPhone 6 not working with UK 'Smarty' network SIM

Hi - I look after an iPhone for my son's use. He had an iPhone 7 which got lost recently. We replaced with an iPhone 6 and ordered a replacement SIM from the provider 'Smarty'.

The iPhone 7 worked without issue on this network AFTER updating the carrier settings to enable data to work as detailed below;

https://help.smarty.co.uk/en/articles/1155220-using-the-internet-after-you-ve-joined-smarty

When I got the new SIM a few days ago and setup the phone I forgot to do the above. My son told me the phone is no longer working outside of wifi. The phone COULD take calls however.

Today I contacted the carrier AFTER setting up the APN details as above. They advised to test in another phone which I did. The SIM DID work in a newer iPhone, and also my Android. As such the carrier are saying its a device issue.

Since I did this testing the iPhone 6 says 'No Service'.

The carrier insist this is a iPhone issue and I'm struggling to disagree..

What I cannot understand is they did confirm a small amount of data DID download on the new SIM so it DID work at one point, and DID accept calls.

I am now faced with either swapping a perfectly good iPhone 6 (unlocked) for another device, or seeking a new SIM provider.

Any suggestions very welcome.