@Sokolov honestly optimize hormones, get your thyroid in order and eat a slight surplus while being reasonably active. You should gain muscle.
For example reasonably active would be chopping wood, moving furniture, taking the stairs always, parking (if you drive a car) at the end of the street and walking longer, if you take a bus or trolley-getting off a stop early or late and walking, cleaning your tub or shower every two days or so, etc. just never slouch on chores, and have the energy to want to do them.