Exactly the same issue here. iPhone 8+
Message said update to 13.5.1 to fix security issues blah blah, which I duly did.
When the update completed, I sat there and watched it, now I have no mobile service.
The security issue must have been to stop people making calls or using mobile data on my phone (including me the security update is that tight).
Restarted phone, removed and reinserted SIM card, reset device. All to no avail.
I put the SIM card into my old Galaxy S7 and I’m back in business, so it’s not the SIM card or service provider.
I’m now the proud owner of a $1000 paper weight.
Thanks Apple. :-(