Need help with a 3 network issue.

[Cricketbat70]

Can anyone help with an issue my son is having with the 3 mobile network?
As a family we all on 3 for our mobile phones, about 6months ago my sons ROG phone 3 suddenly lost mobile signal, he can only make calls when connected to WiFi. Assuming maybe a sim card fault, my son got another sim in January which didn't help, he then renewed his contract, thinking that maybe his contract was that old and gave him a deal no longer offered by 3 was the reason it wasn't working. That didn't resolve the issue.
He then took his phone into a local shop for repair, a shop we have used before for device repairs. He was told by the engineer there was nothing wrong with his phone, they had taken it apart checked the hardware and cleaned the phone, they had tried sim cards from three other networks and all work perfectly.
My son got a cheap gif gaf sim and it works perfectly. His 3 sim does work in all our phones it's just his ROG phone it doesn't work in. My son contacted three who put him through to some network issue department and all they said was "your model of phone is not on a blocked list, your phone needs repairing"
He's now stuck with a contract he'll have to pay £400 to cancel, he obviously doesn't want to keep topping up the gif gaf sim but our question how can sim cards from other networks work perfectly and the 3 issue be a fault with his phone?

 

[Messiah]

Do any of the other 3 SIM cards you have work in his phone?

A quick Google also comes up with:

Based on user experiences, the ASUS ROG Phone 3 may experience issues with the Three UK network due to VoLTE (Voice over LTE) incompatibility or incorrect Access Point Name (APN) settings. Because Three has phased out its 3G network, a phone must support 4G/5G calling (VoLTE) to function properly.

 

[Cricketbat70]

Do any of the other 3 SIM cards you have work in his phone?

A quick Google also comes up with:

Based on user experiences, the ASUS ROG Phone 3 may experience issues with the Three UK network due to VoLTE (Voice over LTE) incompatibility or incorrect Access Point Name (APN) settings. Because Three has phased out its 3G network, a phone must support 4G/5G calling (VoLTE) to function properly.

None of our 3 sims work in his phone. Although an old phone his ROG phone is 5G. How do you change VoLTE and APN settings. Isthat something 3 have to do over the air or can we manually do it.