Computer Hope

I have a Samsung TAB A 10.5 SM T-595 (LTE)  which I bought in February 2019. Since then, I have done several software upgrades and updates without any problem.
However, following a software update in early August I have the following problem.
On starting/re-starting, I am asked (as usual) for my device PIN.
After loading, I was then always asked to enter my SIM card PIN.
However, since the latest software update, this screen either no longer appears or flashes on just for a nanosecond.
The result is that I cannot make any outgoing calls. If I make a call, I receive a message saying 'Not registered on Network'.
Strangely, at this point, after several minutes, I sometimes  get the SIM card PIN screen. The only other way to get this PIN screen is to open the SIM card carrier and hen close it again - the SIM card PIN screen then appears!
I have spent hours talking to Samsung and to my network provider, Smarty. They each say the other is at fault!
This morning, I took my tablet to the Samsung shop in London, Kings Cross for a 30 minute consultation.
I started/restarted the tablet 6 times (to demonstrate the problem) and it worked properly 5 of those times! Came home (London, East Finchley) and the problem started again!!
The Samsung rep said that the latest software update may have been aimed at 4G. He wasn't sure if my tablet can receive 4G ('Connections'  states 2G, 3G and LTE) and it might have something to do with this. But, if this is the case, why does the tablet work ok in London, Kings Cross but not in London N2?
Any ideas?