The rapper's lawyer, Lance Lazzaro, tells TMZ ... his client is going to be part of 4-year-old Saraiyah's life one way or another in a couple weeks when he's released from home confinement. He says the simplest way to make that happen is get a judge involved to hammer out a court-ordered custody arrangement ... despite protests from his ex, Sara Molina.

Lance tells us Tekashi wants to be there for his kid -- and that the only reason he hasn't been since getting out of prison was for her own benefit. We're told it wouldn't have been prudent to have Tekashi visiting Saraiyah these last few months ... you can imagine why.

Sara says if Tekashi really wanted a relationship with his kid, there are many ways to do that ... and he hasn't to this day.

Sara tells us if 6ix9ine wants in again on his daughter's life, he's gotta play by her rules and ease back in as she sees fit ... not on his timetable. With that in mind, Sara says she has no problem if his own mother continues seeing the girl -- which has gone well thus far.

It's interesting because Sara seems willing to crack the door open for Tekashi in about 2 weeks -- but the way his attorney's talking ... it sounds like they're going over her head.