BigHammer: Exactly as sbiddle mentioned, just connect the WAN port of your LinkSys WRT1900AC to your ONT. Done. You're in business. (Note: You may have to update the firmware of the WRT1900AC, as old firmware apparently didn't support VLAN tagging.)
If you need to, think of the ONT as the modem (it isn't, but it sits between the router and the cable coming in to the house, as a "modem" would on a copper line).
Thanks.
Pretty sure I'm on the latest firmware (1.1.8.161917).