At eight months, that may be one of those issues you might have to deal with for a year or more if all other attempts to resolve it fail.
For me I had an ENT take a look a couple of times without any real help until things cleared up literally two years out.
Our healing from radiation is so very slow. The body’s response to radiation is unlike anything else we ever experienced. For many of us, we have symptoms and issues for many years that we either learn to live with or we just get used to these things as normal in our everyday lives. Phlegm and mucus however are not socially accepted public issues. I had at times said simply to annoyed people “Forgive me. I am recovering from cancer treatment.” Then I could turn and hack up a lung with impunity.
Hang in there. Time will pass and you will feel better.