I can activate on RE450 the 5Ghz access point, the ssid shows and I can connect with the PC, but as soon as I connect the RE450 to the main router 5GHz My_Wifi, the SSID PRIVATE_WIFI disapears.
Does someone have an idea of what I did wrong?
I tried to create interfaces and activate them inspiring me from that page, but I am really getting lost
do you want hosts in 192.168.1.0/24 to be able to talk to hosts in 192.168.2.0/24 ?
Not sure if it is best for you but you can do this as router config and not extender mode.
On the RE450, configure 5GHz radio to uplink to your MY_WIFI. The RE450 will get an IP on 192.168.1.0/24. Then use the 2.4GHz radio as normal WLAN with the PRIVATE_WIFI ssid. 192.168.2.0/24 clients just route out the RE450 and then the ISP router. Would be getting double NAT'd by default, but you could turn that off.
If you want the two networks to talk, need a static route on ISP Router to send 192.168.2.0/24 traffic to the RE450's IP on 192.168.1.0/24.