While the scripts (Bengali/Assamese and Tibetan) both evolved from the Gupta script, the actual languages are very different. Bengali and Assamese are Indo-Aryan, while Tibetan is from a completely different language family (Sino-Tibetan).

When I (bengali speaker) visited Bhutan where they speak a language that is 50% mutually intelligble with Tibetan I didn't understand anything. I was surprised because I thought they might use a number of buddhist loan words, but even the words for dharma, karma, etc. sound completely different in tibetan

For reference:

https://en.wikipedia.org/wiki/Bengali%E2%80%93Assamese_scrip...

https://en.wikipedia.org/wiki/Tibetan_script