The probable issue now @ComicSans is that as you have let an unauthorised engineer carry out work on the phone thus breaking the phones seals your Samsung Manufacturer Warranty will be voided.
Samsung wouldn't probably touch it now.
The shop that carried out the work I assune guarantee their work so if they've snapped a part of the antenna off surely they must remedy that.
The conspiracy theory in me says did they also snap the internal board if they've already admitted to snapping the antenna. Nothing you could prove unfortunately.
This is why I advocate always going to the official Samsung Service Centres or Samsung Experience Stores if possible and local even if the cost is higher than a shop that advertises they repair phones.
In regards to the internal boards you would have to look around at different shops ,but then also weigh up the time and costs involved to try and get this phone working again against the purchase of a new phone.
Coupled with what you've paid the shop already for the work carried out thus far.
I assume you do not have any form of insurance you can claim from ?
If this was my phone I'd let it go and buy another phone along with a phone case and screen protector, but that's just my personal opinion.
I wish you all the best with this situation.
Daily Driver > Samsung Galaxy s²⁴ Ultra 512Gb ~ Titanium Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
