Since I've noticed that if you buy premium it may not be accessible in another platform of MC, I thought that maybe when you got premium, you would get a bundle of keys for each platform. Then, when you logged on you would be able to insert the key for that platform. Another fix is maybe when you buy premium, it is put on your account as My Colony premium buyer, so when you log on another platform, the game realizes you've bought premium for my colony and gives it to you.
newstart (United Earth)
erm, buying premium activates premium for said account and works on every platform.
only exception in a way is steam version, buying steam version does give premium everywhere, but if game wasn't bought on steam you won't get steam version(which is premium only) for free, got to buy it again to have it on steam.
That said, if you did buy the premium on one platform and it didn't transfer to other one you should contact bast or wait for him to see this post I guess so he can help with solving that