LFG made my PCP – who said he was not qualified – complete my forms

I have been on STD that progressed to LTD since April 27, 2014 with Lincoln Financial via my employer Moses Cone Hospital. It initially was denied off and on due to wrong dates and information done by PCP, after all that was straightened out they have paid pretty regularly with occasional calls and requests for medical records. Then in December 2015 they required another attending physician statement be done by the same incompetent PCP.. NOT any of my treating doctors. At first he refused to do it because he said he was not a specialist who understood or actually treated my illness. He wrote on the form “unable to complete” and sent it in. I had to go back to him and tell him he was required to complete the form because I initially went to him as my PCP in April 2014 and even though he was not any of my treating specialists he had to do the form. He was irate, sat across the room from me and never did any type of assessment. He said he would obtain some records from one of my neurologist and use their information to complete the form. He wrote that I had no illness, no disability, and no restrictions and was being treated for neuropathy by several other specialists. They also required and FCE be done. LFG continued to pay my claims until April 2016. They called and then sent a letter that I was being paid my benefits until July 2016 and was then released to work on August 1, 2016 based on the attending phycisian statement by my PCP in Dec. 2015. This coincidental date is also the same date the extension of my benefits starts. I want to file an appeal but have NO IDEA what I am doing.