United were swept aside by the League Two outfit during a chastening opening 45 minutes, as goals from Charles Vernam and Tyrell Warren saw the Mariners race into a two-goal lead.
Bryan Mbeumo halved the deficit with his first United goal, before Harry Maguire levelled to send the game to spot-kicks.
Matheus Cunha had the opportunity to send the Reds through after Clarke Oduor missed, but the Brazilian was denied by Christy Pym and a marathon sudden death period ensued.
Fifteen consecutive penalties hit the back of the net, but Mbeumo’s second attempt from 12 yards rebounded back off the crossbar to leave the Reds reeling in the rain.