The honest answer is that it depends entirely on what an hour of downtime costs you — and most businesses have never worked that number out.
Do the arithmetic first
Take your revenue per working hour. Multiply by the fraction of it that stops without internet — for many businesses that is nearly all of it, because payments, bookings, phones and email all ride the same circuit. Add the cost of staff sitting idle. That is your hourly downtime cost.
Now compare it to the monthly cost of a second circuit. For a lot of businesses the backup pays for itself the first time it is used.
What a good backup looks like
The standard design is a fiber primary with a coax or fixed-wireless backup on a physically diverse path, tied together with SD-WAN so failover happens automatically in under a second.
The words doing the work there are physically diverse. Two circuits from two carriers that enter your building through the same conduit are not redundant — one backhoe takes out both. Ask specifically how each circuit enters the building.
Why coax is a good backup specifically
It is inexpensive, and it usually arrives on genuinely different infrastructure from fiber. You are not buying it for its speed. You are buying it for the fact that it is unlikely to fail at the same moment and for the same reason as your primary.
When you probably do not need one
A small office where work continues on phones for an afternoon, with no payment processing, no hosted services and no scheduled deliveries, is spending money on insurance against an inconvenience. Say so out loud rather than buying it reflexively.
We would rather tell you your risk profile does not justify a second circuit than sell you one you will never use.
The middle option people forget
If a full second circuit is hard to justify, a 4G/5G failover appliance covers the "cannot take payments" scenario for a fraction of the cost. It will not carry your whole office, but it keeps the till open.
