@ShaunJR 

Factory reset your router by holding the reset for 15 seconds, when the router boots back up, connect to the network, and test again. Make sure to test while in the same room both 2.4Ghz, 5GHz, and being wired to the router.

If you still see slowdowns, connect directly to your modem to test the speeds as well.

Downgrading the firmware involves going to the firmware section of your router, downloading the previous release, then manually uploading the .bin file you get from extracting the downloaded zip file.

@Tony 

Thank you sir. Is it possible to connect this router to another router and thereby extend the coverage in my house?

How do you do this and which router should be the main one?

@ShaunJR 

With the AX50 you can set one as an access point, then run an Ethernet cable from the LAN of the main router to the WAN port of the one running as an access point.

Here is a FAQ that shows how to do that: link