I'm not able to enter the just bought Realm

I had bought for my son last week a one month subscription for a realm.

I'm running Mincraft Java on a Ubuntu 16 system.

When I'm starting Minecraft and go into Mincraft Realms, the bought Realm does appear, however when I click on play, Minecraft tries to connect for about one minute and then I get a timeout error. I have no idea if this is a problem on my side or a server problem.

I have found that certain ports need to be open, and I have opened them under Ubutu, however this did not change anthing.

I have searched for more detailed tutorials or problem soving for linux but not found anything for realms

I would appreciate much if anybody could help, before the 30 day are over

Cheers