S21 ultra restart issues and poor connectivity after one UI 6.1 update

[Nomeking]

Hello Everyone,

I recently encountered a serious issue with my Galaxy S21 Ultra after i updated to One UI 6.1 on June 22, 2024. Since the update, the automatically restarting randomly upto 10 times a day. I can't interact with my device at all. Sometime it reboots in the middle of important tasks, phone calls inclusive.

To troubleshoot, I have tried wiping the cache partition and even performed a factory reset 4 times, downgrading back to One UI 6. Despite these efforts, the phones still restarts randomly most times with reboots and poor network connection. To make matters worse the moment I get invalid network notification phone is unresponsive and restarts automatically. 

The phone is only 5 days old and it's very annoying to have to encounter such when the device is new.

I'm at a loss and would greatly appreciate any advice or solutions from the community. Has anyone else experienced this issue? Are there any additional steps I can take to resolve this?

If possible is there a way of downgrading to one UI Version i had when i first bought the phone?

Samsung please respond here!

Thank you in advance for your help!

[Sky4ll]

Try the following.

[Nomeking]

I tried clearing cache several times conditions still persistent

[Nomeking]

Nothing had changed maybe I shouldn't have updated the software after it was released

[Nomeking]

Am in Zambia now

[Neolyn]

Try Samsung Dignostics in Samsung Members to find a possible fault. Tap the Support tab in Samsung Members then phone Dignostics. Test all.

[Nomeking]

I have tried 3 different local network providers. the restarts become more frequent when network is lost, and from the diagnostic, as you can see, I have tested several times whilst connected to the internet. The restarts are more frequent with each Sim card when network is lost. I have tried in different areas to test the network, but it's not improving. The Connectivity issue with local network providers resurfaced after the update am now suspecting it has affected mobile network. Because I use the same Sim cards on my galaxy A51 and everything is okay on that device.
Please help further investigation

[Neolyn]

When you purchased the device did they state that the Galaxy S21 Ultra is Carrier or SIM locked.

[Nomeking]

The phone is both factory and Sim unlocked

[Neolyn]

After some research I have a solution for you

[Neolyn]

Go to Settings and Follow this direction.

Settings > General management > Reset > Reset mobile network settings > Select SIM Card > Reset Settings.

After

Settings > General management > Reset > Reset WI-FI and Bluetooth settings > Reset Settings.

Close all apps, and then restart the Galaxy S21 Ultra.

Please tell me if this worked or not.