I'm pretty sure im softlocked out of Multiplayer/Realms in Minecraft because of the region...

I havent loaned him my account, he bought the game with his credit card under my email adress, and used Hong Kong as the region to pay with HK (his bank is in Hong Kong), im unsure if that switched the transaction region, but it was bought in Serbia. Im unsure if that gave my launcher regional restrictions, so im wondering if the best course of action is to just refund the game and then buy it again but pay in Serbia region. He also removed the card info from my Microsoft profile, if that is any helpfull.

If it was a code.then no

As codes.are one time use if you loaned him your account then you need to wait 3months before you can move back

Ill elaborate on the way the launcher was purchased:
We bought the game in Serbia, however my uncle lives in Hong Kong (he came to visit us), so his credit card is linked to a bank in Hong Kong.

This means gave the transactions region Hong Kong, even though it was bough in Serbia. I'm unsure if this gave my launcher regional restrictions on multiplayer and realms.

From Minecraft/Microsoft -

https://minecraft.net/en-us/article/minecraft-multiplayer-server-safety

“Join Multiplayer Games” Settings
If you have your settings set to block joining multiplayer games, you can’t join any multiplayer games in Minecraft, including shared local worlds, Realms or servers. To enable multiplayer in Minecraft, make sure this setting is set to ‘Allow.’

However, just like ASCllnaut, that setting was already turned on for my children. In other words I'm in the same boat, we can play on the LAN but not on a realm.

Here you can find some troubleshooting tips for Minecraft multiplayer:

Minecraft Realms and Multiplayer Troubleshooting