There are no airports in Transnistria, and even if there were, it would be impossible to land there. Even ignoring UA and RO air defences, all of Transnistria is within UA artillery range.
Road transport is obviously impossible without Russia first capturing Odessa, which is clearly not going to happen.
Sea? Any watercraft would be destroyed by anti-ship missiles before getting anywhere close, or get shredded by UA artillery while going up the Dnister.
Ukraine on the other hand could crush resistance in Transnistria within a day or two, there's simply nowhere to hide from artillery fire on that narrow stretch of land.
Of course, there's no point in doing that. There's nothing worth capturing in Transnistria, a power plant and a bunch of poorly stored ammunition that's not going to be in usable condition.