Let your players play
You've got a very interesting and different world. Rather than trying to come up with solutions for your players, let them come up with ways they want to do things and then adjudicate whether or not they'll work.
You can ask the specific methods here if you want another opinion, but your world is up to you and you can reward your players for creative thinking if you like their plans.
If you want these systems in place, you are very free to create them. Given your world is not standard, you are more than free (you are encouraged!) to develop systems that work for you. Trying to force the square peg standard rules into your homebrew situation may not be the best fit and you are definitely free to utilize existing content in new and interesting ways that fit your narrative.
You want Teleportation Circles to be cross-planar? Go for it! You want sending or Sending Stones to work that way - make 'em! Don't be afraid to homebrew mechanics for your homebrew world!