Translation:My mother subscribes to two monthly magazines.
"My mother is subscribed to two monthly magazines" does not work as a translation?
Sounds ok to me as a native English speaker, report it ;). I think the given example is perhaps slightly better than yours though, in that it is more commonly used.
It may have more than one meaning.... it may mean to sign, underwrite, to note, mark out or also subscribe (to)
Congratulations, you just learned a new word in English AND in Portuguese. It cannot be "signs", because in English, you sign documents by inscribing your signature on them. However, you subscribe to recurring events- tv shows, sporting events, and most commonly, publications of various sorts such as newsletters and newspapers.
So in summary, you sign documents - letters and contracts, and you subscribe to publications - newspapers, newsletters, etc
Subscribe is somewhat related in that it is what happens when you sign up for something such as regular delivery of magazines, or season tickets.
"two monthly magazines" is not the same as "two magazines monthly", I can subscribe to two magazines monthly (as in, per month) without either of them being monthly magazines.
Duo seems to be accepting both, when they're not entirely the same thing.
Two magazines monthly = duas revistas por mês. In Portuguese there is also the distinction.