Two bridges and failover?

So, what I'm trying to achieve maybe is too hard for my knowledge, if someone who knows more than me is willing to help...

What I would like to do is using my :

Raspberry Pi 3B+ with :

  • its native ethernet port connected to an Ubiquiti antenna.
    This outdoor antenna is connected to my friend's wifi AP with internet access.
    The device also acts as a DHCP server, but I think that it's my raspberry who'll act as DHCP server for my home network.

  • its native wifi radio, disabled (too bad)

  • USB 3.0 to Ethernet port, to connect the raspberry to my LAN switch.
    I found the openwrt drivers for the usb adapter already.
    On this switch i'll connect all the devices and my local AP, and use the raspberry as gateway for all my internet traffic.

  • USB Wifi Antenna
    To have a failover internet access, it will be connected to my smartphone being an LTE hotspot.

Speed is not really a concern, but I'd really need that if the Ubiquiti connection doesn't work, the failover will be faster than 20 seconds.

I'm trying to use MWAN3 and RELAYD, but I'm failing hard.
How would you try to achieve something like this?
Thanks anyone!!!

I suggest that you start simple, test first and then you can look into things like MWAN3.

Based on what you said, there is no need for Relayd.

It's better to keep the Ethernet port for your LAN switch. That way, something wrong happen to the USB adapter, you still have access to the Pi. There might also be speed advantage in that setup (reported by some users for RPi 4).

It makes more sense to have the RPi as the/a DHCP server. You haven't provided much about the Ubiquiti device and weather it has an adequate firewall or not, but presuming it doesn't, then you would likely want your RPi to act as a router, creating it's own subnet (in which case it doesn't matter if the Ubiquiti DHCP server is working or not.