@FernandoZago 

I got the same issue after update the FW to the lastest version (1.1.7 Build 20201010 rel.49458); my internet connection dropped to 30~80Mbps on wifi (2,4 and 5 Ghz), but on cable i got full speed.

After reinstalling the FW three times (even downgrading it) and reset the router a couple times, I assumed that the problem was in the Wireless settings. So I changed the Security from WPA2/WPA3-Personal to WPA/WPA2-Personal and the issue was solved. The wifi connection was able again to reach full speed from my ISP, like the PC ethernet port.

My ISP's modem provides 4 gigabit lan ports and I use an Archer T9E on my PC (2 meters away from the router, no barrier), always connected up to 866,5 Mbps, so it was not an issue on my PC side. Even because till the FW upgrade i got 0 problems like this.

I am very sorry that an official file from the manufacturer could cause this kind of situation. Not the end of the world, but some waste of time to solve. 

I hope a bug fix anytime soon.

@IWannaSeeUBurn 

Humm.. that's odd.

I didn't notice any difference.

Have you checked if there is no other device stealing your network bandwidth ?

Have you tried transfer a file between cabled device and a wireless device and check the speeds ?

I very often transfer files between an old notebook and a pc (that is on cable). I a really didn't notice any speed drop.

Just tested now with the WPA2/WPA 3 Personal... No difference at all in speeds.

I tried 2.4Ghz (old notebook) and 5Ghz (on my phone)