So this printer is wireless linked to the DIR-615 router. Then, it is a direct networked printer. Steps to make sure it works always:
1) Set up IP address into the Lexmark printer network configure. Printers need to be always found so it should not get IP address from the router's DHCP service. DHCP service "can" give out different IP address.
2) Install the Lexmark printing software system onto each / every PC. Follow the install guide for network connection (Yes, wireless and wired network connection work the same way.)
3) Configure the software firewall(s) to permit the Lexmark software to access thru, accessing your home network.