WiFi Client and AP at the same time, how?

Ahoy friends!
Currently i got this setup.

My network is connected using WiFi to another WiFi network, in order to provide internet connection.
But my smartphones are not capable to get WiFI connection because of the long distance to the other network.
The signal strength is on max. according to LuCi, but my devices can't get WiFi connection to this network.
So i would like to enable an WiFI Acess Point as well on my OpenWRT device, but is this possible to use it as client, and AP at the same time?
I tried this https://openwrt.org/docs/guide-user/network/wifi/relay_configuration tutorial, but when i try to setup Master and Client and the same time, one of them is always deactivated.

Both SSIDs must be on the same channel. Set the Master (AP) WiFi to Auto


It may be a possibility that your hardware does not support multiple SSIDs and that is why it shuts down WiFi when multiple SSIDs get enabled. Although what you are trying to achieve is possible under OpenWrt but may not be possible for your hardware if it is not supported.