If the Fritz router runs pretty much to its default settings I would expect
A) the router to be 192.168.178.1 (subnet mask 255.255.255.0 and gateway(!) 192.168.178.1 too)
B) this means your network devices (when using static IP settings) have to adjust: IP such as 192.168.178.x (x=2-254), subnet mask 255.255.255.0, standard gateway 192.168.178.1 (or whatever IP the Fritz router has)
If the ZTE router had a different IP and subnet than the Fritz, and you did not adjust the NUC’s network settings your problems make sense.