I am wondering if you have any other symptoms of any kind that have gone along with this muscle weakness. Examples- severe fatigue, joint or muscle pain, hair loss, anything at all even if it seems unrelated. Have your muscle enzymes in blood been checked? These are specific tests that are not usually ordered. (CK, aldolase). These are elevated in muscle inflammation (myositis) which can cause muscle weakness and atrophy as the only symptom. It’s uncommon and may not have been thought of. General inflammation markers may also be helpful to provide a clue to cause. (Sed rate, CRP). I would get a copy of labs you have had and look for those or ask Dr if they have been done.
Doing what exercise your can is essential to prevent worsening and hopefully will improve your strength but I would not give up on searching for the cause. Have you seen a neuromuscular specialist who works with nerve and muscle disorders? That’s essential.