How can you return synchro monsters from your graveyard to the field?

You are allowed to use Monster Reborn and similar cards, under most circumstances. The important thing is that the Synchro Monster must have been properly special summoned first. If the Synchro Monster ever goes to the graveyard without being properly summoned by Synchro Summon (ie, Starlight Road can do this) then the Synchro Monster cannot be revived like that. Similarly, if a Synchro Monster had its summon negated, it also can't be brought back from the graveyard.

Last thing to consider is some might say "cannot be special summoned except by Synchro Summon" meaning the only way to summon it is by Synchro Summon, there is no other way, even if it was properly summoned first.