What follows is a bit of a brain dump on how I went about implementing mobile failover for my FTTC connection. I hope it is useful to others thinking of doing the same.
I've had a Netgear AC762S mobile hotspot for a while. Netgear finally released the DC113A Ethernet cradle for their latest series of mobile hotspots last week, so last week I ordered a DC113A cradle and a AC785-100EUS SIM free 4G hotspot to replace the Netgear AC762S.
This equipment is on sale at Amazon today - the DC113A is £39.99 and the AC785 is £79.99. This is a total of £45 less than I paid. By threatening to return the items and re-order at the lower price, Amazon customer services agreed to refund £45 to my card.
I use a Three contract mobile broadband SIM in the hotspot - 10GB for £15 per month, SIM only, 30 days notice. This works with the 3internet APN, which gives you a dynamic IP address, no NAT.
The AC785 uses a micro SIM rather than the mini SIM of my old AC762S. I got a replacement SIM from a Three store rather than cutting down my existing SIM.
The Ethernet connection on the DC113A cradle is bridged to the LAN side of the AC785S. You can obtain an IP address using DHCP, though I chose to set the Ethernet interface of my router to a static address outside the AC785S's DHCP pool.
I set my router to 192.168.1.128/24, gateway 192.168.1.1. I then turned on the AC785S's DMZ feature and set it to 192.168.1.128, so all incoming traffic is sent to my router. Using my router's dynamic DNS feature, I configured a host name that is set to the mobile IP address when the FTTC connection goes down.
The AC785S's DMZ feature is transparent to every protocol I've tried. I've currently got a Hurricane Electric IPv6 tunnel active over the Three connection. With further configuration and testing, it should be possible to have active IPsec VPNs fail over from FTTC to mobile by using IKEv2 with MOBIKE.
For an outlay of £119.98 plus shipping (nothing if, like me, you have Amazon Prime) for new hardware, together with a mobile broadband contract I had anyway, I now have automatic failover from FTTC to mobile on an FTTC failure. If I need to take the mobile broadband with me, I pull the hotspot off the cradle and close the covers over the antenna connectors. You can 'hot dock' the hotspot when it is turned on if you wish.
I have the Wi-Fi on the hotspot set to switch off after 15 minutes of no connection, so the Wi-Fi eventually switches off when the hotspot is docked. You can set the AC785 to disable Wi-Fi and/or pass through the mobile IP address without NAT if you USB tether the hotspot, but neither of these features are available if the hotspot is docked in an Ethernet cradle. Hopefully Netgear will enhance the AC785 firmware to provide these two features when docking the hotspot in an Ethernet cradle.
The AC785 is a considerable improvement over my old AC762S, not least as the AC785 has dual band Wi-Fi. The only significant AC762S feature that is not implemented in the AC785 is the micro SD card based NAS, but I never used that feature anyway.