This is from Google AI so don't put too much trust in it
Yes, Cambodians can file their 90-day report online in Thailand. Foreigners staying in Thailand for more than 90 days are required to report their address to immigration authorities every 90 days, and this can be done in person, by mail, or through an online system.
To file online, foreigners need to access the Thai Immigration Bureau's website and use the online service for reporting over 90 days. They will need to have their passport and arrival card (TM.6) information readily available, and the online application must be submitted within 15 days of the due date, according to Acclime Thailand.
Give it a try and see what happens. As has already been pointed out, do it sufficiently early so that you still have time to visit your immigration office if necessary.