Can't add Guest Access Portal from cnMaestro to my Master Controller AP

Could I please ask how you have set the devices up?

Has it been done through AutoPilot? See here as an example: https://community.cambiumnetworks.com/t5/cnPilot-E-Series-Enterprise-APs/AutoPilot-How-to-Onboard-and-Configure/td-p/75253

Or has it been configured as a standalone device? See here as an example: https://community.cambiumnetworks.com/t5/cnPilot-E-Series-Enterprise-APs/Version-3-2-GUI-on-E400-E500-makes-things-easier-to-get-to/td-p/69329

I'm just trying to work out how your network is currently configured then I can tell you how to onboard it to cnMestro to use the cnMaestro guest portal.