This is a separate issue. I think you might posted the wrong thread, most active thread is below. But long story short additional resolutions will come through a kernel sync with AML, but that was not part of tthis testing thread of moving to Debian stretch.
Debian 9 is just indicating the repository base. All these devices will use different kernels with different support for DVB. Sam backported most drivers but there might be an issue with your specific one. I suggest to first search the forum if someone before already report issues with your stick otherwise open a new thread as this thread is about the switch from Debian 8 to 9.