You’re likely a victim of this recent update:

For me, both the 2ghz and 5ghz have slower speeds and intermittent drops. Interestingly, the 2ghz is faster at ~50mbs down vs the 5ghz at ~15mbs. I have random ping spikes and disconnects on both.

In attempting to narrow down the issue, I’ve tried all the a/b/g/n/ac/ax settings. Specifically when we to ax, none of my devices are able to connect to the 5ghz (but still can to the 2ghz). This used to work fine, and was the reason I bought the g36 as ax is wifi 6e if I’m not mistaken. 

Before the update, I was getting 300+ mbs on 5ghz with a silky smooth connection. After, and currently, it’s borked 90% of the time.

I tried a full reset - as in pressing and holding the tiny button in the back until it reset the whole thing. It took maybe 10 minutes and wiped all of my wireless settings… but this method also fixed everything for me! I cannot explain why - the settings are back to defaults but all of my devices can connect and I am getting 200+ mbs on the 5ghz. Very few lag spikes.

I highly recommend anyone that’s having wifi issues with the g36 take the time to do this. It’s annoying, but I’m going on 12 hours now with no issues. I’ll definitely come back to report if that changes.

Since you are experiencing with intermittent internet connection or the Wi-Fi network disappearance, I suggest you to check the firmware version.

Also you can change the Wi-Fi channels to the least congested one and monitor the performance for the next 24hours of time.

To change the channel, find the link given below:

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Channel

And make sure the at DCS scan interval should be 8hours.

Thanks to the mod post for having me remember to report back. Just a few days after the reset the wifi started acting up again: lag spikes and disconnects on 5g. Then a day later, 5g disappears. Now 2g only is up but it is also having lag spikes i.e. awful wifi performance.

Seems like the arris g36 surfboard is just a failed product, or we're all the victim of some extremely poor manufacturing or cooperation with Xfinity. Whatever the case, it's time to return this and avoid Arris in the future.

Could you let us know the firmware version for the G36 modem. That helps to check for further assistance.