A printer at every place that makes a drink, driven from the counter beside it, with paper, power and a spare you have already tested.
On a busy bar the printer is not an accessory to the register. It is what tells a pair of hands what to pour. Staff ring the order in at the register and take the payment there, and the ticket prints where the work actually happens. So the first decision is not which model to buy, but how many places on your floor make a drink or a plate: the main bar, the service well at the quiet end, the kitchen that sends food on the same order. Each of those is a station, and each station that makes something gets its own printer.
The setup that fails on the hardest night is one printer behind the middle of the bar doing all of it. Tickets for three different pairs of hands come out in one stack, somebody has to sort them, and the sorting is where an order goes missing, exactly when nobody has a spare hand. Splitting the order in the system instead - one ticket per station, each carrying only its own lines under the same order number - costs nothing once the menu knows which station owns each item.
How the printer is fed matters more than the badge on it. The printer beside the register should be driven from that counter, so an order rung in there does not have to travel out to the internet and back before a ticket exists. Venue wifi is shared with everyone standing in front of the bar, and bar power is a run of extension leads, so both will dip. What you want is an order taken during a dip to still be taken - recorded somewhere durable, queued, printed when the link returns - rather than refused at the register or lost between the two.
That durable record is the whole safety net. A paid order that only ever existed inside a printer cable is an order you cannot recover. Written down first, it comes back by itself once the station has paper and power again, with nobody retyping anything, and an identifier tied to its station means a retry produces one ticket and not three.
Set the printers up as they will stand and ring real orders at them. One item for each station, then a mixed order across three stations, checking each ticket shows only its own lines under one order number.
Then break it on purpose, while the room is empty. Pull the paper out mid-order, finish the order, reload, and watch the missing ticket arrive on its own. Send the same order twice and confirm one ticket at each station. Unplug a station and see whether anybody is told, and on which phone. Take the wifi away for a minute, ring an order, and watch where it surfaces. A setup that comes through those four is one you can leave behind the bar on your busiest night.