Diagnosis
The ET-2850's wireless connection can be flaky, especially after a router reboot or if there's interference. The printer's network settings sometimes get confused or drop out.
How to Fix It
Start with the printer. On its screen, press the home button, go to Settings, then select 'Wireless LAN Setup'. Run the 'Connection Check' tool. If it fails, select 'Restore Network Settings' to wipe its wifi memory. Then, you'll need to set it up again like new using the WPS button on your router or by selecting your network name and entering the password on the printer screen.
If That Didn't Work
The issue might be your router assigning it a problematic IP address. Try giving your printer a reserved IP address (a static IP) in your router's admin settings. You'll need the printer's MAC address, which you can find on a sticker on the printer or in its network status report.
Heads Up
The printer's wifi chip isn't the strongest. If it's far from the router or buried in a cabinet, that can cause chronic dropouts. Moving it even a few feet can make a difference.