Hello @Wouter_Heerwegh welcome to Zwift forums.
Some disappointing news first, then some really exciting news for you.
Up until 2023 - smart trainers were designed for road bikes and their taller gearing. Just as you would spin out your mountain bike’s biggest gear on flat roads outside - you could also spin out your trainer because your gearing wasn’t big enough. This applied to all direct-drive trainers, by the way.
The good news is that the Zwift Hub you own is capable of virtual gearing, and can overcome the physical gear limitation on your bike. More information about Virtual Gearing when we introduced it to existing Hub Classic owners such as you. This FAQ might also be useful.
During the first few seconds after your avatar spawns, Virtual Gearing will calculate what physical gearing is on your bike, and set your trainer in the 12th of 24 virtual gears. After that - you leave your derailleur alone and shift virtually using the Zwift Play controllers, or the Zwift Click. Both are available on the Zwift Shop, but for mountain bikes - you’ll want the Click and ZwiftCog combo. The Play is designed for drop handlebars and will not work with mountain bikes.