I had horrendous problems when iTunes showed an update to 17.4.1 (which my iPhone already had!) was required and I tried to update. I ended up calling Apple Support around 8 times and spending 4 days trying to recover my iPhone and restore all apps and logins. All of this seems to have stemmed from the re-release of 17.4.1 which couldn't be updated via the phone, only by iTunes. I'm extremely unhappy that I've wasted 4 days down to what appears to be a bug in the update process.
I thought the discrepancy between my iPhone and iTunes version was a syncing problem so I updated via iTunes and the update crashed a long way through the process. My iPhone (which had been fine before the update) wouldn't boot up properly and required resetting via a PC. Despite connecting it to iTunes, it would not boot up and I couldn't do a reset or restore on it.
After multiple calls to Apple, I had to use another PC, download Apple Devices and try it all over again. Eventually, this did reset my phone BUT it wouldn't connect to the mobile network - the signal kept appearing and disappearing and I could see that the menus under Cellular Data were constantly changing as the phone appeared to be constantly trying to connect to the mobile network and failing. Internet worked OK but not mobile calls/data.
By using another SIM and another iPhone, I confirmed that my SIM worked in the other phone but another SIM wouldn't work in my phone.
After further multiple calls to Apple support and remote diagnostic testing (and a superbly helpful Irishman!), I managed to reset my iPhone to a clean version and then restore to an iCloud backup.
Unfortunately, whilst all my apps were downloaded, I had to re-login and setup every single one which took me a further day.
The whole incident has been a nightmare and caused me serious issues and loss of time.
It was only my persistence with Apple Support and my belief that my phone was fine and it was the software update which had corrupted it. Apple Support repeatedly said they couldn't do much more and to take the phone to my nearest Apple Store (a 4 hour round trip) for testing which wasn't sensible initially when it seemed as though a straightforward reset/restore should be possible.
I must thank Apple Support in Ireland and in North Carolina who were very helpful in problem solving at various stages - Ireland, in particular, spent well over an hour on the phone to take me through all of the reset/restore stages when it failed initially and they had the ingenuity to get me to use another PC and another SIM to problem solve.
Apple - PLEASE take more care with software updates and advise customers if there's a discrepancy between phone/iTunes updates and why - then they can be more circumspect about updating and decide whether to delay if there are continuing user issues.