Opnsense Connection to X55 Deco Mesh WiFi Setup??

[Jssnow87]

I'm new to this process as well. I have the x55 Deco Mesh WiFi system, it is currently connected to my isp modem. I know I have to put it in AP Mode when trying to connect it to my opnsense router. My question is when?

Do I switch to AP Mode now and then move over the Ethernet cable to the opnsense router, or do I connect the opnsense router to my isp modem first and then bring over the Deco Mesh last and then switch it over to AP Mode?

I need help with the steps to make sure I still have wifi access. Any help or questions would be greatly appreciated.