What is a 4G or 5G failover router?

It is a router with a mobile SIM alongside the fixed connection, which switches over automatically when the fixed service drops:

• The switch is seconds, not a phone call - staff often notice only that things slowed down.
• It covers the common case - most outages are the fixed line or the exchange, not the mobile network.
• Costs are modest - the hardware once, plus a small ongoing data plan.

Pick a mobile carrier that is not the same network as your fixed service, or you have bought one path twice.