Thank you for your feedback.
I wanted to extend the aerials and put them outside (use this Delock Antenne LTE model).
But do I have to do it for both or is one enough?

Changing a single antenna might help depending on your setup. But I would say replacing both antennas is recommended for optimal performance and stability. It’s generally better to install a dual-antenna (MIMO) setup if you’re upgrading the existing antennas with an external one. If you are replacing your antennas because of lower signal quality at your current router location, your new antennas should be selected accordingly. I mean just replacing antennas per-se might not do magic to your received signal quality. In this case, you can use new antenna's with higher gains (dbi) in the relevant frequency band (4G: 800/900/1800/2100/2600 MHz). Then place them accordingly towards the signal source. If you cannot change the location of your device, you can consider using an antenna cable extender to place the external antennas to a different/better location.