Wie kann ich mein T-Mobil Surftstick auf OS 10.11.6 (El Capitan) update?

(Bitte um Entschuldigung, dass ich dies auf englisch schreiben musste.)

I travel to Germany every year. Since there's no wireless where I stay, in the past I've used a T-Mobil Surfstick.Last year I got a 2015 Macbook with OS 10.11 (El Capitan, not yet upgraded to Sierra). When I went to my local T-Mobil store with my new computer, however, I was told that I would need to update my software on the new computer before I could use the Surfstick again. However, supposedly I need to consult Apple about how to do this, since the T-Mobil tech couldn't tell me where to find the correct software.

This seemed like something T-Mobil should be able to help me with. Fortunately, I now have an iPhone 7 that I can use as a personal hotspot. Still, I have the German device and it has non-expiring data minutes on the SIM card, so I'd like to be able to download the software that would allow me to use it. (Which is way less of a hassle than trying to get SIM card money back from my local German T-Mobil store.)

Does anyone know whether and at what URL I can download the right software for OS 10.11x? (Telekom website or Apple?) Any help would be much appreciated--thanks!