Your form/subform will not work for many to many relationship.
Apparently I am not able to adequately explain what you must do but I will give it one more shot.
tblPatients
PatientID (primary key)
etc
(do not put ProviderID field in tblPatients)
tblProviders
ProviderID (primary key)
etc
tblPatientProvider
PatientID (foreign key)
ProviderID (foreign key)
Set main form RecordSource to tblProviders.
Set subform RecordSource to tblPatientProvider.
Have a combobox with unfiltered RowSource listing patients from tblPatients to select existing patient to associate with provider.
There is a way to show all the related patient info in the subform but could not use this form to add new patient record.