Use of 'rhag'

As I understand it the Welsh word ‘rhag’ means ‘from’ in English. Can it be used to say …

Dw i’n rhag Casnewydd - I’m from Newport

It occurred to me that if the syntax is valid it negates the need for a nasal mutation. What is your opinion please?

It’s not a matter of opinion. Rhag does in fact mean “from” in certain cases, but not in this case. Words rarely map one-to-one from one language to another, and this is the most obvious in prepositions, and it takes a long time to develop a “feeling” for the right preposition, so in the beginning the best bet is to learn them one by one in connection to the verb.
I come from … → Dw i’n dod o … (with soft mutation).
(One example for rhag is “to escape from …” → dianc rhag …)

Dwi derbyn a deall. Diolch.