In theory, if there are 2 WiFi networks having same credentials then its more likely that your devices would connect with the one having the strongest signal regardless if the internet is accessible or not from there. So you'd have to turn off the non-internet WiFi network to force move all devices to the internet WiFi network.
Regarding the option of mobile hotspot, I'd say it might be suitable for some browsing, email or streaming on probably 1/2 devices. But anything higher than that it's very likely to struggle a lot because the hardware (i.e. mobile hotspot in phones and usual 4G dongles) isn't similar grade like dedicated routers and doesn't have their reach. Still you could give this option a go and see how it works out for you.
If you're unsatisfied then you could go for a dedicated 4g/5g sim router, which should give you the connectivity and WiFi reach like your existing VM router. There may be PAYG options here as well so you'll only pay as you need when the internet goes out on the VM router.
Though keep in mind that if you keep the WiFi credentials same on 2 routers, the above issue of strong signal/WiFi hopping would still surface if both routers are powered on at the same time. The best way to get round this would be to keep the WiFi router separate from the ISP router.
The way this would work is that you get a dedicated WiFi router and turn off the WiFi on your existing ISP router. This WiFi router would then connect with single cable to the ISP router (giving it internet access) and in-turn all your devices would connect to this one WiFi network. So now if the internet goes out, you can simply switch the backend cable of the WiFi router from the VM router to the 4G/5G router instead and all your devices get internet connected without WiFi hopping or having limited connectivity.
