So if the MX are actually up and running, then the issue might be simple.
Go to Security & SD-WAN > SD-WAN & Traffic Shaping > Uplink statistics and add a new destination
Change it from Google to something else that you know you can reach. Change the default to the new one and uplink status should reflect that.
I had an MX once sitting behind another firewall that was blocking Google DNS, so it was showing 100% down even though it wasn't. Had to add in 1.1.1.1 instead