@David-TP First pic shows the said features turned on,

Second pic shows I'm currently connected to the Guest Network

Third pic shows guest network can access my Main Network's Router GUI OpenWRT (My DECO m5 is operating in AP Mode)
Fourth pic shows the speed limit in download and upload that I set and,
Fifth and last pic shows the Bandwidth Control isn't working.

Was running my Deco M5 in AP mode since I bought it, and was using the "Isolate from Main Network'" and "Bandwidth Control" feature since the latest firmware update introduced it specially the Bandwidth Control one. It was working before then I decided to randomly test now and it shows otherwise.

  @David-TP I understand now that Guest devices can access the Main router GUI, they should just be isolated from each other and the devices from the Main Network. AP Isolation if I'm not mistaken. :)

Regarding the Bandwidth control problem, I did reset all 3 Decos to factory to eliminate all potential set device/application priorities to start from scratch, unfortunately after setting it up and turning on Guest Wifi and setting the BW Control again, it still doesn't work

  @David-TP Received and responded on it! :)