This is getting a bit OT
I started the thread and I'm not going that route anyway, so it's OK with me.
Regarding using 3G/LTE as a backup at home I have been wondering about how to do this - I have several PCs at home and not all of them have Wifi cards so going all Wifi isn't really an option for me. What I would be looking for is a way to replace the WAN connection from Rogers cable with a 3G/LTE connection while keeping the rest of my LAN infrastructure in place.
I imagine I could tether the phone to a PC and then bridge this connection to my regular LAN connection.
The other way to do it is the way that you are suggesting I guess where you would use a wifi router as a wireless bridge with the Wifi connected to a phone which is a wireless hotspot and the wired part of the router connecting back into the rest of the LAN. I should try this and see how it works as it should be pretty simple to do.
If you had two Wifi cards in a PC you could also have one Wifi that is your "WAN" connection to your Phone/iPad which is in Wifi hotspot mode and the other Wifi connected to your LAN and bridge the two Wifi connections. In either case I guess you would have to make sure that everything is on the same subnet.
I think the best way would be to use the phone as a hotspot, then connect the router to it using WDS/Bridging etc. Using a PC tethered to a phone then used as a wireless hotspot onto the WLAN then back onto the LAN sounds overly complicated, but should be possible. It's just that PC wireless hotspot software tends to suck
at least in my experience, and getting a router to connect to a phone in hotspot mode should be easier.
The only downside is automatic failover. Most of these routers can save their configuration and reconfigure themselves from a saved file, perhaps you could save a normal configuration file then a backup 3G/4G configuration file and switch between the two when required? That's the beauty of ASUS's arrangement with a 3G/4G dongle, failover was automatic.
Firmware with complete access would allow a script to be written that could do this, "then it's just a simple matter of programming" as they say.
You would have to install an alternative firmware that allows root access and scripts - like Tomato or DD-WRT on the RT-N66U.