One little notice, in the future it can be useful. 

   Now that we know that use to is out of question, how to say "Eu costumo jantar fora com frequência" then?.

   The words "dinner/diner" and "out" seem like don´t fit in the same sentence, at least most of times.  This I gather empirically from the hits I got crawling within Google.

   Thus,

"I use to have dinner out frequently".

"Eu costumo jantar fora com frequência".

   Instead, you would translate   "Eu costumo jantar fora com frequência" into

"I frequently go out for diner..." or         "Frequently I eat out..."       

    It might have other ways of saying this, but I will leave it at that.

  By the way; in Portuguese it would be expressed ''Eu janto fora com frequência/eu janto fora frequentemente/eu frequentemente janto fora".   

   Since, costumo  e frenquência in the same sentence turns it into a redundant one.

   At least in my thinking.

+++++++++++++++++++++++++++++++++

    Examples of "dinner" and "out" in the same sentence (but, not with the meaning of eating out):

-Instead they kept it simple, with a dinner out Saturday night at an elegant, American-fare restaurant near Georgetown

-Diner-out:
One who often takes his dinner away from home, or in company.
A brilliant diner-out, though but a curate.

-thanksgiving diner-out.

PP.

   The answer is open to comments, corrections and additions (not to be confused with

addictions, he he).