'HSM' Star Monique Coleman on Zac Efron's Absence from Disney Sing-Along

Monique Coleman says there's a perfectly good explanation why her old Disney Channel costar, Zac Efron, didn't join in on a big broadcast sing-along ... the almighty Wi-Fi signal.

We talked to the 'High School Musical' alum -- who was one of the former child stars in the hit DC movie series, and who performed "We're All In This Together" Thursday alongside Vanessa Hudgens, Ashley Tisdale and others -- and she told us why Zac was MIA during the nationally televised performance. Long story short ... dude's probably got a bad signal.

Monique says Zac -- the lead man in the 'HSM' movies -- got on-board at the very last minute, and was only able to introduce the song from a fuzzy video he sent in. The reason he wasn't able to sing along with the crew is because he's likely in a remote part of the world with crappy Internet. Last she heard, ZE was in Papua New Guinea filming for his new Quibi show, "Killing Zac Efron" -- so yeah, he wasn't gonna risk a bad show.

More importantly than that, though, is the fact that Zac is one of the most down-to-earth celebs there is in the biz ... according to Monique, anyway. She says she's not surprised he didn't cave to the Disney pressure to get in on the sing-along ... and did what he realistically could.