Text/photo messaging with Android users

The "Send as SMS" is meant only when you are unable to send iMessage. That settings does not activate or deactivate SMS. If you have an SMS plan enabled by your Carrier for the SIM that you use then you can send SMS in the absence of iMessage else no.

Similarly, if you have an SMS plan enabled by your Carrier for the SIM that you use then you can send SMS to Android users otherwise no.

You may not have an SMS plan enabled by your Carrier for the SIM that you use on your device. Apple Inc does not provide SMS services nor can it be enabled or disabled on an iPhone.

This necessarily have to be enabled by your Carrier for the SIM that you are using.

Send SMS/MMS | Email & Messaging | iPhone 12 - T-Mobile

T-Mobile Internet and MMS settings: Apple iPhone

Have you tried resetting your network settings? Settings>General>Transfer or Reset iPhone>Reset Network Settings. The only thing you'll lose is saved WiFi passwords.

But, in the end, this is a TMobile issue. You'll need to keep escalating the matter with them. Keep in mind that, even if you select "tech support" when you call them, the first person you'll be connected to is a customer service agent with a bit more training and a set of scripts. They're goal is to get you off the phone as quickly as possible. You'll need to keep insisting on getting moved to Tier 2 tech or whatever TMobile calls it.

Best of luck!