Message not delivered after 15.2.1 update

I have iPhone 12 Mini and using VI prepaid sim card in India.
Till today morning I was able to send and receive SMS (from non-iPhone).
I saw that 15.2.1 update was available, so I downloaded and updated my phone.
After update, now I see SMS is not getting sent. It is displaying Red exclamation mark.
I tried disabling iMessage, tried to turn ON/OFF WiFi and mobile data, but still same issue.
Any fix expected?

On your iPhone, go to Settings → Messages
Scroll down to the “Send as SMS” option and then make sure the toggle is enabled.

Thanks @Just_Me_D
But I don't see "Send as SMS" option in settings.
Since iMessage activation is not working, so I had disabled iMessage.
I am able to send SMS since yesterday.