The sentence "Can you help me on with the engagement ring?" is grammatically correct, but it might sound a bit unusual or awkward. Typically, we use the phrase "help me put on" rather than "help me on with." So, a more natural way to phrase this request would be:
"Can you help me put on the engagement ring?"
Alternatively, you could say:
"Could you assist me in putting on the engagement ring?"
Both of these variations are more commonly used and would be easily understood.