I also have this router AX120v2 and have added latest beta firmware installed

[baneand]

I also have this router AX120v2 and have added latest beta firmware installed which has fixed random rebooting (so far 2 days working good). But I have 5GHz wifi problem - occasionally it just doesn't work, when I leave my pc and get back to it, I see the internet is not working even though wifi connection is still active. I fix it by reconnecting but it is disappointing to have to do that frequently for router this priced.

[baneand]

Another thing noticed - wifi gets stuck approximately once per minute or two - I just simply listen to some music files over 5ghz form my NAS device and the streaming gets stuck for couple of seconds. I was testing 2 different wifi 5 antennas I have, still waiting my wifi 6 to come but I don't expect it to behave differently. Again software issue hopefully in beta firmware. 

[baneand]

Posted on another topic but here might better place - I am using the beta firmware (ending in .43-rc) and that has fixed random reboot problem but wifi gets stuck approximately once per minute or two - I just simply listen to some music files over 5ghz form my NAS device and the streaming gets stuck for couple of seconds. I was testing 2 different wifi 5 antennas I have, still waiting my wifi 6 to come but I don't expect it to behave differently. Again software issue hopefully in beta firmware. Not sure if anyone had this issue to suggest a solution.

[FURRYe38]

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?
Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended. 

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and 40 to 48 channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

Has a factory reset and setup from scratch been performed since last FW update? A complete pull of the power adapters for a period of time after the factory reset then walk thru the setup wizard and setup from scratch with a wired PC and web browser. Recommend setting the default DHCP IP address pool range to the following after applying and a factory reset: 192.168.#.100 to 192.168.#.200.
https://kb.netgear.com/24089/How-do-I-specify-the-pool-of-IP-addresses-assigned-by-my-Nighthawk-rout...

[baneand]

My internet modem is Cisco EPC3928, I am using it for last 8 years with another asus router without problems. Cable is was CAT5 but from what I tested these days only wifi is problematic. I switched now to CAT6

Channel on 5 GHz is 44,

I did try factory reset just now and set the range of ip address from .100 - .200 but same behavior - basically communication drops each minute or two for couple of seconds and then it gets running good again. 

[FURRYe38]

What happens if you disable AX mode on the router and also disable Smart Connect? 

What are the current settings for Wireless settings under advanced tab/advanced settings? 

Whats the first 3 characters of the routers series number in back? 

[baneand]

First 3 chars are 6K0, I did disable AX mode now and Smart connect was already disabled. Checking later advanced settings too

[FURRYe38]

AT some point, please try with Smart Connect enabled as well. 

[baneand]

Unfortunately tried the smart connect but same behavior. Also noticed that after some idle time, me away from pc for half hour or something like that, the communication over 5ghz is completely blocked. Then I just reconnect and it works again. Not sure if there is anything more to try before I get a new router.

[FURRYe38]

Possible you have a faulty router and needs to be RMAd. Something to check with NG support to see what your options are. 

You could disable the wifi radios and install a different wifi system, something like a Nighthawk MK or Orbi MESH system in AP mode.