I also have the same problem as yours however, I am not the clan commander. My friend told me that the reason why I got accidentally removed from the clan is due to linking my gamertag to a new or other COD account at callofduty.com website. I did link back my gamertag to my original COD account (the one I used when I first joined the clan) and it resolved my issue. So, I got back to my clan. I hope this also works for you since I also shared this one to my friend who is also a Clan commander and it worked to him.