Is your topology as below?
FTTP Router(LAN)---(WAN)C9>>>WDS via 5GHz<<
Try WDS via 2.4GHz.
Firmware of your C8 and C9 are both up to date?

Firmware was up to date on C9, but not C8. Just updated it. Topology is exactly the one you described.

Now I cannot even make the bridge via 5Ghz. On 2.4Ghz there is no packet loss, but I'm getting lower Mbps, 120-130 instead of the 180-190 I got previously with AC. My FTTP connection is 300Mb/30Mb, so it's kind of a big loss.

I don't know what's going wrong with the setup on the 5Ghz band. I changed IP and disabled DHCP on the C8, just as I did yesterday, rebooted, but there was no luck this time around. Also, something weird happens every single time: no matter what channel I chose on the C9 (right now 157), when linking, the C8 will say it needs to change the channel, but it always detects a different channel than the one I configured on the C9. I haven't got a clue of what's going on. On 2.4 Ghz, it was easy as pie.

Thanks for your help :)

PS: Needless to say, but using 2.4Ghz standard is not what I was looking for with this C8/C9 setup. 2.4 Ghz band is way oversaturated in my network, with 6 to 10 devices connected to it, so substandard performance is to be expected.

EDIT: SOLVED!!

Had some missconfig the second time around on the C9. Now it's getting 240-250mbps and 0% packet loss, with just 1ms jitter. Yei! Seems like C8 firmware update really did something. If only the UI was prettier, a little more clean and had less complicated configurations, this would be kind of perfect routers.