Cannot connect the default OpenWrt 5g network after connecting OpenWrt router to my ISP router

If you're connecting to your ISP device as a client, set the AP to auto.

This is because the AP needs to know the STA channel first- most importantly, the radio can only use 1 channel, hence why the AP doesn't come up.

If you need channel 36 to be statically set, try setting that on the ISP device.

There's the Travelmate package that may assist also.

Hope this helps.