Network found, but fails to connect

Since just before Christmas my phone does not want to connect to my provider network anymore. When I do a manual search I can see the network is there, but it fails to connect. As my provider did not want to help as I’m running a unsupported OS, I’m trying to figure out if I get some message back from the process to connect. Where can I see what the reason may be for failing to connect?

Is your phone 4G/5G capable?
Did your provider abandon 2G or 3G?
In what continent are you?

Phone is 4G capable (XA2).
Maybe 3G got abandoned, but I’m not able to connect to 4G either.

I was curious to check on my XA2, both on 4.3.
They both have a similar phone operator plan.
One of them stays in 3G all the time while the other uses 4G without problem. (same settings)
I didn’t notice the difference as the 3G is still in use here in France.
To have a scientific comparison, I should compare both with fresh flashed OS.
But still, both should have 4G.

It is not the network itself, 3G is working on the Blackberry. But asof December 21 I’m not able to connect.

Is there some file I can look into to see whether or not I receive some message back when trying to connect to the network?