it is a brand name and should remain in English or should be transliterated

I think you've left out a S! Isn't it "Fast Plants"? I visited its website, and saw that wherever the trademark (®) has been used, the phrase includes a S.

And don't forget that it is فست not فاست, as we have فست فود!
Moreover, you had better include its English name in parantheses after it for the first time you transliterate this phrase.