It depends on what sort of threat you're designing your contingency for. For example, if you're worried about deauth attacks or someone jamming the parts of the RF spectrum used by your WiFi network, having a backup network won't help if it's vulnerable to the exact same attack.
You'd be better off using a wired network for the really critical systems, and either accept the risk that you might lose the wireless ones or use something more robust than WiFi (such as 5G).
In general, relying on wireless communications for critical applications is problematic. Protection against jamming attacks has been part and parcel of military communications systems since World War II, but this protection is never really considered in civilian systems (protection against unintentional interference is a different issue).
This is for good reason: compared with confidentiality protections like encryption, properly protecting against jamming attacks requires very severe trade-offs in system performance, and those trade-offs just aren’t justifiable in civilian systems which aren’t intended to withstand a military threat.
For example, strategic military satellite communications links—designed to withstand intense jamming attacks and still provide reliable nuclear command & control—have data rates measured in kilobits per second. Even if 802.11 or 4G LTE are (by comparison) hopelessly vulnerable to jamming attacks, that sort of speed decrease isn’t a compromise anyone in the civilian world would be willing to make. Jamming is a risk, but it’s a risk which isn’t worth mitigating.
Another factor in this equation was that jamming historically required specialised hardware of the sort only the military had easy access to. It’s also very illegal to jam commercial communications systems in most countries, which made manufacture of this sort of hardware not worth the effort even if you wanted to try it.
These days, however, Software Defined Radio has massively decreased the barrier of entry to anyone who wants to play with radio signals, and it no longer requires thousands of dollars in specialised equipment if you want to do something (again, very illegal) such as jamming or spoofing nearby GPS receivers.
(Some of the above paragraphs paraphrased from here)