Help: 2.5Yr old back to waking up at night

[Biggie]

My 2.5 yr old used to sleep thru the night till 7am. Since we return from Phuket 3 weeks ago, he has been waking up at least once in the middle of the night and then at 5am. He sleeps in his own room and would cry out for mom when he wakes up in the night (There's a baby monitor in his room). I have to go to his room and just sit there till he falls asleep again. If I ignore him, he just keep crying and screaming -- I haven't tried to just let him cry for too long.
Not sure if the trip triggered this. We were in Phuket for a week where he was sleeping in the same room as us, and would climb into our bed when he wakes up early (like 4am). We let him cos it was a new environment and thought he was insecure. Also, we didn't want him to cry and wake up baby brother (8mth) who was also in the same room.
Now back at home, the baby brother sleeps with us so if he comes into our room crying, baby will wake up. Or, if he sleeps in our bed, baby will wake him up (baby also likes to cry at around 5am). So no one in our family (not even our dog who also sleeps with us!) can get a good night sleep!

Any experience to share to deal with toddler sleeping issues?

[Obiwan]

Yes, reckon the waking is triggered by the recent Phuket trip. It happens to us, often after a long trip. After each trip, we will sleep train our little one again, usually takes 1 to 2 days of CIO (if jet-lagged, will take a little longer) and she is back to her good old ways again :)

[Biggie]

Is 2.5 yr too old for CIO? I know during the day he can cry for a looong time. Plus he can actually get out of bed and open the door of his room .....
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[Obiwan]

Ah my daughter is slightly younger so she can't open the gate we have at her door .....