Thoughts on IPhone 6s not working with new sim?

[tight fart]

I’m trying to put an elderly relative on to a cheaper sim only deal with ID mobile.
ID have sent the sim but it won’t work in this old phone, currently working with a Virgin O2 sim.
O2 say the phone isn’t locked to them, the iPhone settings say it isn’t locked.
The sim works in our old spare phone (Huawei p20) and the ID sim from the P20 doesn’t work either in the 6.
I’ve tried, turning off on, airplane mode, travelling to different masts etc.

Neither ID or O2 have been able to find a solution.

[cobra kid]

Give the elderly relative the P20? I gave my P20 to the mother in law. She's in her late 80s and uses it fine.

[CSR Performance]

I had loads of issues 12 months ago with an ID sim in an iphone. Eventually solved by them issuing me a new physical sim which my contract etc was ported over to. I think I had to go to a nearby Currys and they did it in store, then took 12 hour or so to switch over.

[GuyW]

Given the phone is 12 years old and so far behind in terms of iOS versions, it can't be used with any apps I'd imagine. Only as an actual phone and texts? If so, I'd look to get something a bit more modern. I know iPhone 6 was pre 4G, not sure if the 6S is. Possibly this could be an issue?

[wyson]

iPhone 6s is 4g. Will it work with a 5g simcard? I remember back in the day, my mobile provider being very particular about this when I got a sim only contract. They wanted to confirm if I had a 4g or 5g phone so they could send out the correct sim card.

[Dogwatch]

The 6 is no longer supported (since last Summer).

Herself had one (refurb) and the battery failed pretty much on the day support discontinued which was handy as she was reluctant to switch to the 11 I had got her from CEX. She's fine with it now and can continue with her important WhatsApp chats with The Daughter.