I activated wifi but can’t change it to channel 9. When I select channel 9, it remains on channel 7.
I suggest you check that the correct country is selected first. I ssh’ed in to the router and did:
root@GL-AR750:~# iw reg get
country GB: DFS-ETSI
(2402 - 2482 @ 40), (N/A, 20), (N/A)
(5170 - 5250 @ 80), (N/A, 20), (N/A), AUTO-BW
(5250 - 5330 @ 80), (N/A, 20), (0 ms), DFS, AUTO-BW
(5490 - 5710 @ 160), (N/A, 27), (0 ms), DFS
(57000 - 66000 @ 2160), (N/A, 40), (N/A)
Channel 9 is in the first band 2.4 GHz, which, here in the UK (Great Britain == GB), is 80 MHz wide.
Now if your router is trying to broadcast a 40 MHz wide signal - say 300 MB/s Maximum, then there are only two channels here in the UK. Other countries vary so look it up on Wikipedia if you want to know.
My router has set itself to channel 6. The channels are 5 MHz apart. Thus we only have two non-overlapping channels which are 40 MHz wide. If the signal is only 20 MHz wide - for 150 MB/s there are three non-overlapping channels these are usually 1,6 and 11. 9 should be possible with this 20 MHz width, but is not possible with 40 MHz width because the signal would go outside the allocated band.
So how do you set the channel? The best answer may be to let the router decide for itself. I’ll let someone who knows say if there is intelligence in this router to select the best channel. These days it depends on the interference received from other routers in your vicinity.