Short answer: check that the printer and your computer are on the same network; Microsoft notes that access points, extenders and extra routers with separate network names can split them. Restart the printer, and if it’s still offline, remove and re-add it. If it keeps dropping after the router restarts, a reserved IP address may help (see the last section).
Check the printer is on the same network
Microsoft says to make sure the printer is connected to the same Wi-Fi network as your device, and Apple says to check that the Mac and printer are still on the same network, the printer is on, and the network isn’t having problems.
Microsoft adds that if you use access points, extenders or multiple routers with separate network names, your computer must be connected to the same network as the printer. Apple notes that if a printer doesn’t appear in the list at all, it may be turned off, or your Mac may be on a different network now.
Restart it, then re-add it if needed
On Windows, Microsoft’s offline checklist includes a full power cycle: turn the printer off, unplug it, wait 30 seconds, then plug it back in and turn it on. It also suggests setting the printer as default (on Windows 10, clearing Pause Printing and Use Printer Offline), and removing and reinstalling the printer if it’s still offline.
On a Mac, if the printer doesn’t appear in the list of printers, Apple suggests trying to add it by its IP address. If you do, reserve that address in the router too, as below, so it doesn’t change.
Stop its address changing
Routers normally give devices an address that can differ from one time to the next; a reservation fixes a device to one address. ASUS and NETGEAR both let you reserve an address for a device by its MAC address,.