https://dentalchat.com/
Ask questions and chat with dentists using the online form or app
https://denteractive.com/
The free tier allows you to send photos and get a reply from a dentist within 4-6 hours, app available
https://askadentist.com/
Online question form promises a response within 24 hours
https://www.theteledentists.com/
Paid online teledentistry service
https://www.icliniq.com/ask-a-doctor-online/dentist
Online medical consultation with a dental section - free question using the online form for the first query, app available
https://www.justanswer.com/dental/
Broad ask-an-expert website with a section for online or phone chat with dentists
https://www.reddit.com/r/askdentists/
If you don't mind your question (and photos if you choose to upload them) being visible to the general public and are aware that anonymous people not verified to be dental professionals may respond to you with possibly incorrect advice, ol' Reddit has r/askdentists - not necessarily recommended