Can't erase, pair, restore Apple Watch

Hi There title says it all. I'm trying to get my watch working after stupidly pairing it with a new iPhone SE while travelling. I've now unpaired it and gone back to my old iPhone 6s.

Have tried...

• Fully erasing the watch (twice)
• Updating the iPhone to iOS 15.5
• Followed various help articles like this one and this one

Issues...

• The old iPhone recognises there's a watch to setup
• It recognises the weird security animation
• It sees there's a backup of the watch and starts to restore
• But after about 1min it fails

I'm travelling so am connected to a mobile network but this can't be the issue because I can stream live TV with no issues. I've also just download about 18GB of Photoshop files and upload 6GB while tethered (not while restoring).

How do I fix this?

I'm loathe to reset the phone because I need it for work . Any help would be much appreciated.

Cheers, Ben