As the picture shows if the "ISP router" goes down or some problem arises with the connection it uses the network will become disconnected. I'm wondering if it's a viable option to add the MX67 to supply a backup internet connection via the cellular of the MX67 for the MX80 at the site as suggested in the picture. This would be done by converting the 2nd Lanport of the MX80 to a wanport and connecting it to a lanport of the MX67.
Note that the network is currently operating as the picture shows besides whats contained inside the "Suggestion" box.