S20 Ultra voicemail and data/wi-fi issues

If there are any employees monitoring this forum, I would sure appreciate a response from them as well as any help I can get from other members.

Purchased S20 Ultra for Christmas from Samsung after being told by both Xfinity AND Samsung that the unlocked version would work just fine (since Xfinity would not come close to matching Samsung's price).

Set up the phone, backed up old phone, transferred everything with SmartSwitch and all seemed to be well while I was at home for a week and not using the phone for anything much except calls.   Then once I started getting out again, all h*ll broke loose.

1)  Previously transferred saved voice mails disappeared and the Xfinity Mobile VVM app disappeared.  I know now that it's not compatible, but the built-in Samsung VVM app is supposed to be, but when I try to open it I get an error message (9007) instead of it asking me to log in with user ID and password on the first attempt.    I've seen people on here saying Xfinity employees got their voicemail app working, but hours on the phone with Tier 1 and 2 support (and Tier 2 support saying they've messaged back and forth with Tier 3) only resulted in them blaming Samsung and saying there's nothing Xfinity can do.

2)  Wi-fi and mobile data do not want to stay turned on.  Their icons in the drawer show that they are on, but there's no connection when I try to access the internet unless I'm connected to my home Xfinity wi-fi, and clicking on the icon to see more details reveals that the toggle for mobile data (or wi-fi) is actually OFF even though the icon says it's on and I manually keep turning it on.

3)  All text messages copied over just fine (back to 2014 even!), but the thread where I text myself with reminders and important info I don't want to forget when I'm on the go disappeared sometime after the initial setup.  I checked once the phone was set up, to make sure Photos, Texts, Voice Mails etc. all transferred over.   Now when I try to text myself and hit "send", it just keeps trying and the message is never received.

Please help...... I don't want this phone if it's  not going to work properly.   Of course, when I confirmed that everything transferred over and the phone seemed to be working ok initially, I sent in my old S8+ (which admittedly was acting up - hence the upgrade) for the trade-in credit.  So I can't even go back to my old phone.    

TIA!

Brenda