Can a POS system keep working without an internet connection?
Yes, but only if it was built that way. MB ALFA is offline-first: every till holds a complete local database, so cashiers take orders, print receipts, split bills and close shifts with the network entirely down. Queued work syncs to the server in order when connectivity returns, without duplicate orders. Systems that merely cache a menu will let you browse offline but not sell.
The distinction that matters is between offline-tolerant and offline-first. An offline-tolerant system keeps a copy of the menu so the screen still renders, then fails at the moment you try to take payment or close the shift. An offline-first system treats the local database as the source of truth and the server as a replica.
The test to run during a demo: put the device in airplane mode, take three orders, apply a discount, split a bill, close the shift, then reconnect. Everything should arrive on the server exactly once.