S21 5G automatic mobile network select does not switch to optimal available networks resulting in bad performance.

[NyleFLandas]

I have an S21 5G with T-Mobile, if I turn off automatic network selection for the mobile network and select only the T-MOBILE network, I get coverage over 99% of my major routes and most of it is 5G. If I instead turn on automatic network selection, the phone switches over to mostly AT&T towers at 4G and offers virtually no connection for roughly 50% of my main routes. 

Three related questions -

1.) Is anyone else seeing this issue?
2.) Why is automatic network selection not reconnecting to my primary provider when their network is offering a far superior/stronger connection and instead leaving me connected to another carriers network? (I've even toggled airplane mode on and off - it stays on the low signal other carriers network.)
3.) I'm willing to leave automatic mode off but is there a way to stop the phone from popping up constantly asking if I want to select another network? (As I said T-Mobile coverage is actually very good on my major routes.)

Thank you.

Side thoughts:  (It appears that AT&T is bandwidth throttling, even when the signal is strong. Could they also be locking the phone to their tower or is this a phone issue?)

[Tt16]

I have the same issue for several years with different Samsung smartphones: in automatic mode, the device remains very longly on the  same network, even if it is very weak.  So the communication is bad, for voice and data. Moreover, the manual research is slow and can last 5 minutes !

[Jiricek98]

I have the same problem with my S21 5G.

When I go to another country (from Czech Republic to Poland), I have very weak LTE signal for a few kilometers, because I can still reach Czech carrier

However, once Czech carrier is out of reach, I get no signal. It takes up to 20 minutes for the phone to switch to the PL carrier (or it does not switch network at all).

If I connect to PL network manually, it works in under three seconds.

Just a little technical data:

I use T-mobile, unlimited everything + 20 GB roaming. 

My phone got new motherboard and literally 70% of insides after a warranty claim (and still is under warranty) 

[jk747]

I am having similar issues when travelling from The Netherlands to Germany; data connectivity is lost. (Vodafone). When switching to manual network selection i am selecting O2, but network cell info lite APP keeps saying I am.connected to VF GSM; so no mobile data. There is definitely a bug in the Samsung S21 regarding network selection and roaming under weak signal conditions.