Is it possible for one to use OpenWrt to have the following:
-Cable Internet Line goes down >>> Failback to a mobile phone's WiFi hotspot which stays on 24/7 as ab back up.
Pre-requisites:
-Router acts as a Station
-Router also acts as an Access Point (broadcasting a wireless connectivity as normal).
One way to accomplish the switching is the multi-WAN package.
There may be some "challenges" with running as an AP and client at the same time, on the same radio. Generally, the AP won't "come up" if the client configured hasn't connected yet (the "thinking" being that it doesn't know which channel to use).
Does your phone support USB tethering and charging? That might be worth considering, depending on your use case (as it would mean the phone would need to be "stationary" at the router).
Would this solve that problem?
Well, this may provide some insight. (No, don't click that link, really, you've been warned!)
The current link for AP+STA mode is on the current wiki, https://openwrt.org/docs/guide-user/network/wifi/ap_sta
(Yes, full links are fine to post, especially when they indicate what is on the other end)
https://openwrt.org/docs/guide-user/network/wifi/ap_sta#step_3revert_to_ap_only_mode_on_boot_when_hotspot_is_not_available looks like it addresses the "AP up when STA not connected" problem (however, rereading it looks like only at boot time).
The travelmate package handles this as well, and it might be worthwhile to look into its source to see how it is handled there.
Yes! Sorry about that with the mystery link 
I figured a hover over would reveal the hidden... but probably better form to just put it in there. Will do that in further replies.
I'll check out the new wiki you posted.
Didn't click your link by the way. 1... I was afraid to and 2... I try to follow directions for the most part.
Any other ideas?
I'd explore the travelmate package and see if it has some features that either fulfill, or come close to meeting your needs. You might need to "tweak" it a tiny bit to work with a wired connection, but I haven't looked into it in enough depth to know if/how it handles a wired, upstream connection.
P.S. You're right, a hover would have revealed it on a desktop browser, but not on a phone or tablet
Didn't think about that with mouse-less portable devices... You're right!
I think the route I'm going to test out is to have the main router use WAN (wired internet) and have an LTE Mobile device feed WAN2 as a wireless (WWAN) connection using the router's WiFi as a client.
As far as home WiFi broadcast (AP), rather than trying to dual purpose one router's radio, I'll run a cable to another router and use it's WiFi as an AP. Seems to be the simplest solution. I have a spare router so that will be no problem.
Now to figure out how to use the LTE connection ONLY as a failover... that's the real challenge. I'll have to go over the MWAN3 rules.