You cannot use return ring to warp to an outpost your nation does not have control over. I tested this before I made my OP and i tested it again before i made this response (literally at this very minute.) I am standing in Rolanberry fields, I have every outpost in the game, when i try to use my return ring it says "That action cannot be used in this area" The region is currently under Windurst control, i am from Bastok. I am going to zone into Sauromugue Chmpn. and see if it works, which is currently under Bastokan control. *2minutes later* My return ring worked in Sauromague chmpn, which is under Bastokan control. Thank you and good night.

I have also used the homing ring technique to gain all my OP's and OP warp from my Jueno HP. It was just convenient that Sandy usually controlled the surrounding regions from Jueno. There are 4 regions that connect with Jueno, so odds are your nation will control an exit region.

That being said... yes, it is true that you can only use the rings to OP warp to outposts controlled by your region.