Wireless WAN on a VLAN

In case my cable internet goes down i want to have my whole network to access the internet via a hotspot on my android phone.

OpenWRT is installed on my wifi AP Ubiquiti UniFi nanoHD and thus is only for wireless connectivity. The rest of my network is handled via a pfsense router, and both the pfsense router and the OpenWRT AP are connected via a smart switch (Brocade/Ruckus ICX7250) which means i should be able to use VLANs.

My plan: On the Unifi AP running OpenWRT i create a wifi network in Client mode that connects to the wifi hotspot from my phone (which means the phone is the DHCP server) and make this network available on VLAN 100 on the single ethernet port that the AP has.
Then on the pfsense router i create an additional WAN interface on VLAN 100 that is only used as a fallback when the WAN (cable internet) is offline. And on the switch i configure VLAN 100 for the ports of the pfsense router and the wireless AP so they can connect to each other via VLAN 100.
Oh and of course the AP additionally also serves as normal wireless access point on both wireless radios.
I know that i need to manually enable the hotspot on my phone manually each time and i also would enable the wifi Client network only when needed so the AP would not constantly scan if it finds the hotspot.

I know how to configure the VLAN on pfsense and on the switch.
But i can't figure out how to make the wifi in Client mode accessible via LAN on VLAN 100.
All the guides on the internet are for the reverse case where the wifi is in Access Point mode. I already tried creating a VLAN as eth0.100, creating a bridge device for it and using that for the wifi in Client mode, but it would either not connect to the hotspot or get no IP.

You can't do this directly. Doing a wifi > wired L2 bridge is not a suported function in 802.11 wifi standards. The methods to do this are a bit of a workaround (this includes products that are purpose-built for doing this exact thing -- they're kind of hacks, but they can work).

Your two main options are a standard L3/routed config where the wireless uplink is a different network relative to your ethernet connected network, or relayd which is a hack to operate as a sta-mode device and then provide an ethernet downlink.