John Mousinho’s visitors went into this New Year’s Day fixture still on a high from their dramatic win against Charlton on Monday, but after a period of early pressure the wheels quickly came off.
Pompey had barely got out of their own half when a cleared Twine free-kick fell to midfielder Randell on the edge of the penalty area.
He took the ball down on his chest before firing a swerving shot into the top-right hand corner.
As Portsmouth tried to get themselves back into the match, a pass from keeper Schmid was cut out by Memeti who charged goalwards, and his deflected shot found its way into the net.
The second half did not bring a reprieve for the away side as a Neto Borges cross landed on the boot of Twine, whose sumptuous volley was nestling in the back of the goal before Schmid had hit the ground.
Substitute Sam Bell then latched onto another wayward pass from Schmid and played it to Armstrong, who managed to squeeze his shot under a retreating Portsmouth defender.
Portsmouth’s best chance of the match came just before the 70-minute mark when forward Mackenzie Kirk had a free header saved by Max O’Leary from six yards.
Armstrong, who has struggled for form since joining the Robins in the summer of 2024, was then found in the box with his back to goal, but he turned and smashed the ball into the roof of the net.