@Martinski, Did some more testing on this. Some additional commentary.
Synopsis: It appears when Guest Network is activated there is initially a noticeable WiFi disconnect of approx 2-3 minutes. Without YazFi installed and with Guest Network enabled there doesn't appear to be a noticeable disconnect of the WiFi when accessing and leaving the Adaptive QoS page or seeing an error when running the Internet Speed test under the Adaptive QoS section. With YazFi installed and Guest Networks active however there is noticeable WiFi disconnect of 2 to 3 minutes when accessing and leaving (or running a speed test) in the Adaptive QoS page/section.
What appears to be happening is the Adaptive QoS page is, for some reason, triggering the firewall restart script added by YazFi, then when Guest Network is enabled and when YazFi gets run, it is combining to introduce the noticeable 2 to 3 minute drop of the WiFI that otherwise doesn't seem to happen on the Adaptive QoS section when either YazFi isn't installed or Guest Network is disabled. Its possible this issue has been there for some time and I never noticed it prior to moving to using WiFi only on my main laptop PC recently. (Note: just my wild guess/speculation.)
What follows are my quick notes on what I saw/did just in case it helps:
Did a hard factory reset and initial QIS setup. Do not see a noticeable WiFi disconnect when going to the Adaptive QoS page and running a Internet Speed test, nor do I see an error on the speed test page (like prior posted screen capture show).
The /jffs/scripts folder does not contain any files at this point.
Installed YazFi via amtm. After YazFi install did not do any configuration of YazFi. Did not setup Guest Network. The /jffs/scripts folder has 6 files in it including
firewall-start. Went to Adaptive QoS page and did a Internet Speed test. There did not appear to be a noticeable WiFi disconnection.
Activating Guest Network and inputting the information (SSID, set authentication method, and input password) into Guest Network #2 2.4Ghz and 5Ghz sections, then applied the changes. It was then I noticed a wifi disconnect of approx two to three minutes.
When WiFi came back, I went to the Adaptive QoS page and then returned to the System Log page. After a few seconds the WiFi disconnected for approx 2-3 minutes. Once WiFi was reenabled I went to the Adaptive QoS Internet Speed page and ran a speed test. A few seconds after speed test completed WiFi dropped again for approx 2-3 minutes and the speed test page indicated the error per the previously posted speed test screen capture posted previously.
I next went in and disabled Guest Network(s). Each time I selected the disable button the WiFi was lost for approx 2-3 minutes.
After disabling Guest Network WiFi, accessed and left the Adaptive QoS page. No noticeable WiFi disconnect.
Used the YazFi CLI to remove YazFi including removing configuration files. After YazFi uninstall 5 files were left in the /jffs/scripts folder (dnsmasq.postconf, firewall-start, openvpn-event, service-event, services-start). The firewall-start file did not have the YazFi line of code in it, only text in file was "#!/bin/sh".
With YazFi uninstalled, reenabled Guest Network #2, there was a noticeable drop of the Wifi, again approx 2-3 minutes when each band (2.4Ghz and 5Ghz) was enabled.
Once WiFi returned with YazFi uninstalled and Guest Network #2 enabled, went to Adaptive QoS page and back to System Log page. No noticeable WiFi disconnect. Went to Adaptive QoS and did a Internet Speed test. No error on speed test page, no noticeable WiFi disconnect.