The Bruneian side took a commanding four-goal lead before two consolation strikes from their opponents blemished an otherwise superb home performance
A bleak opening 10 minutes saw two Azwan Salleh free-kicks pose little threat on Joey Sim's goal, with the home side's frustrations underlined by a wild long-range attempt from Sairol Sahari.
The sultanate's only professional club, who had dominated proceedings but had little to show for it, should have broken the deadlock in the 12th minute when Portuguese striker Joao Moreira muscled his way to the byline, but his perfectly-placed low cut-back was mishit by Adi Said from six yards out.
DPMM squandered a second gilt-edged chance when Adi Said cut in from the left wing, and Khayrulhayat Jumat's weak challenge prodded the ball into the path of Azwan Salleh after the 25-year-old had made a well-timed run into the Geylang penalty area. However, Azwan's shot across goal not only had Sim beaten, but also the Geylang upright as the ball trickled out for a goal kick.
The Eagles must have been counting their lucky stars as they went into the break still on level terms, but they eventually buckled under the intense pressure their hosts were putting them under in the second half.
Moreira picked out Helmi, whose pace and energy had been causing full-back Jalal all sorts of problems, with a lobbed pass into the box, and the 26-year-old produced a striker's finish, beating Sim at his near post for his first goal of the season.
It wasn't long before the second goal arrived. Subhi Abdilah's cross into the box was met by Moreira, whose header back across goal looped over Sim and into the top corner.
Stephane Auvray and Awangku Fakharazzi would both have a significant impact on the game, but for one of them it was for all of the wrong reasons.
For Awangku, it was a match in which he came of age and lived up to his potential, which he capped off with an absolute screamer 10 minutes before the end.
Hendra Azam threaded a through-ball to the 24-year-old, who cut inside his marker from the right, and curled the ball over JSim with his weaker left foot, leaving the third-choice Singapore national goalkeeper without an answer.
Moreira added a fourth with a low drive from outside the box, before Auvray gifted Geylang their first goal with a horror back pass to Wardun Yussof. That was intercepted by Yasir Hanapi, who dinked the ball over the stranded 32-year-old.
Auvray was again to blame for the Eagles' second after he clumsily barged Norihiro Kawakami inside the DPMM penalty area. Jozef Kaplan sent Wardun the wrong way, but the two personal errors were blemishes on an otherwise flawless DPMM performance.