Hello, and thanks for your question.
Lymph nodes may be swollen for different reasons, such as different kinds of infections (bacterial, viral, fungal), cancer, reaction to inflammation in the body, to name a few.
I would recommend having your veterinarian perform a needle aspirate of the lymph nodes to see what kinds of cells are present, and if they can determine the cause of the enlargement from the cytology (which is not always possible without the trained eye
of a pathologist looking at it). The next thing (and likely the most helpful) would be to have a lymph node biopsied to send to a pathologist to tell exactly what is going on. They can then determine the likely underlying cause for the enlargement, and possibly the most appropriate treatment. Also, if bloodwork has not already been performed, I would recommend that to look for systemic problems that may be causing the enlarged lymph nodes.
Of note, I have seen quite a few dogs with lymph nodes in the head/neck area enlarged from bad teeth or infected tooth roots (sometimes only diagnosed with dental
I hope this helps.