This is going to include quite a lot of information and will end up being a text wall. Read this all the way through before replying, as most of the things that will be suggested to me have already been considered, done, or are not available to me.
I bought my S21 ULTRA (Unlocked) on 03/26/2021. I was using it on Verizon for several months with no issues outside of Verizon's awful coverage in my area. Last week (08/18/2021) the phone started to randomly shut itself off after dropping cell signal. I have the same exact thing happening with my AT&T sim as well, even though I have much better signal strength. From what I could tell I was not having overheating issues (I have see that floating around as a major issue of the entire S21 lineup), with my highest core temps being around 60C (should not be an issue for modern silicon to operate at that temperature), battery never exceeding 30 or 40C.
I started digging around trying to find solutions on my own, here is what I tried:
Airplane mode on then off
Reset Network Settings (Probably done this about 100 times now)
Clear Cache (Via the bootloader menu - where you power down the device then power it on while holding the volume up button w/power)
Reseating the Sim
Cleaning Sim card and Sim Tray/Slot (I have an anti-static blower made for cleaning electronics)
Factory Reset (Both in the settings menu and in the bootloader menu)
Nothing had worked, so I finally broke down and made the drive to my nearest authorized Samsung repair center (about an hour away). They "factory reset" the device (at a deeper level than what the user can do through software). That seemed to have fixed the issue. My drive up there had the device losing service multiple times and restart once, had almost perfect signal the whole way back. This was 2 days ago (8/23/2021).
Yesterday the device restarted itself unprompted and started dropping signal again. This was all while being virtually right beside a cell tower for my carrier, so signal strength would not be the cause.
Today I have been pretty much unable to use the device without being on wifi.
I have no issues with wifi connectivity. At all. When cellular works, I get expected speeds for my area.
I will note that I am not on the most current software for the device. I will be updating after I post this to see if Samsung issued a software patch for this issue (I am currently operating under the belief this is some software issue and not hardware).
I haven't had any sort of overheating problems with the phone. I've seen plenty of stuff floating around about that and I haven't seen it on my device.
I was wondering if anybody else is having this issue, and if so how many people might be affected...
Kinda defeats the purpose of a phone if it can't keep cellular connectivity!
This is just a thought too, but maybe try running an anti virus, that type of behavior can be due to a virus (although fairly rare but doesn't hurt to do it)
If anything you're still under warranty and should be able to claim that.
Thank you for reaching out. I can certainly understand how frustrating this can be and would be happy to assist you further. The next thing I would try is your device in safe mode. If your phone is misbehaving, it's possible that a third-party app is causing the issue. To figure out what app is acting funny, you can use the Safe Mode feature. In Safe Mode, your phone won't run any third-party apps. This allows you to easily identify and remove the app that may be causing the problem. Give this link a try: https://www.samsung.com/us/support/answer/ANS00062983/
Be sure to click " ✓ Accept as Solution" when you find an answer that works for you.
Please note, notification emails are a DO NOT REPLY address, you must log-in on the community page in order to respond.
Safe mode did nothing for the issue - it was one of the steps I tried prior to taking the device to a Certified Repair Center (the step I had forgotten to write down as well). I tried uninstalling everything manually and through factory reset(s) and was still having the same issue.
The only thing that had any effect was running the phone in "Power Saving Mode" 24/7 and that still had the phone doing the same thing. Also defeats the purpose of having a flagship if I have to run it at 70% SoC capabilities.
Regardless, my phone is on its way to Samsung now. Hoping they can figure out the issue.
A good 2 years ahead of you on that and yes, it does support some of the bands on the towers in my area. Like I said in my original post, it was happening regardless of cell signal strength. I could have signal < -90dbm and it would start saying no SIM or Network not Found.
On that note - I bought a backup phone and am using the EXACT same SIM so that verifies that it wasn't a SIM issue.