TPlink RE450 config as wifi repeater and separate wlans: HELP!

Hello all,
I modified my explanations since yesterday

I am quite new to openwrt.
I flashed my RE450 and it starts well.

The goal is to create a second Wifi network (private_wifi on the image), MY_WIFI already exists

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 to be able to talk to hosts in ?

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 Then use the 2.4GHz radio as normal WLAN with the PRIVATE_WIFI ssid. 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 traffic to the RE450's IP on