I have a really specific issue relating to the Spark 3G shutdown, on a brand new phone. The telco is playing "someone else's problem" so here I am looking for advice.

Phone is a brand new Xiaomi 15 Pro, purchased from PBTech after the 3G shutdown. It support VoLTE, 5G, and NZ bands.

The issue is this:

• Use the phone for a bit, say 24h, on wifi, no issues

   

  • OR using the phone on mobile data for a while, the issue occurs also
• Disconnect from wifi and notice no cell bars and a little x next to them. No connection of any type.
• Go into settings, preferred network, and regardless of what is chosen, pick either "prefer 5G" or "prefer 4G". The phone instantly connects to whatever is available (typically 4G). 
• Wait another roughly 24h, and the cycle repeats.

This is in an urban area with good 4G coverage. It's not a "low signal" type issue as the cell bars are simply empty, and another similar phone on the same network is connected with full cell bars.

What has been tried so far, with no change to issue:

• New SIM card from Mercury, who the phone plan is with (they are a Spark reseller)
• I have 2 Mercury SIMs in 2 phones, tried swapping the SIMs and the issue doesn't occur for EITHER phone
• Accessing the phone diag menu by dialling *#*#4636## and reviewing the diag info found there
• Multiple hours on the phone with Mercury, including someone who I believe was a tier2 tech, mostly going round in circles

My theory: the phone is somehow defaulting back to 3G connection, network isn't there and so it just loses connection entirely. Make sense or not?

What I haven't yet tried:

• taking the phone to PBTech where I bought it, explaining the issue to them and asking for a remedy - I suspect they will say a telco issue. 
• The "reset all network settings" option - it feels like a nuclear option as it will reset all saved wifi networks and BT pairs and so I'd rather not, but if it will solve the issue I will bite the bullet

Any suggestions to offer on what could be the issue, and suggestions to solve?

TIA