Bridged AP full wireless

A quick run down of what I'm trying to do.
I have an old router with OpenWRT installed. I want to have it in the middle of the house so that the other side can get better signal. I can't run a cable to it because of 1. My parents and 2. I don't have a cable long enough.
I want to have the router connected over 5GHz and extend the 2.4GHz. I won't be extending the 5GHz network.
I have been reading the guide on the website for a Bridged AP but those show running an Ethernet cable to the device. I'm not sure what to change to get it to use the wwan from the 5GHz network.
Any help would be greatly appreciated.

If the devices support it...

If you're connecting to a non-OpenWrt router use a routed client instead. This is simple to set up by defining a station mode wifi interface and attaching it to the existing WAN network instead of an Ethernet port. The downside is is only Internet access, there isn't a total bridge to LAN devices attached to the other router.

The device is in a spot where the ethernet ports won’t be able to be used. I just want to extend the wireless. Do you have a guide or like to where i could look more into this?

check this: