Not sure if change steam DL zone could matter for you then its a cart issue
but why not try or test this, cart work fine in my zone.

Ps
No one here can fix your ISP congested lines, if you cant find or get another angle to a steam server that might be better for you, with even DL and i can only hope same goes with cart traffic also improve, if not you can try off peak hours in your zone to see if that help.

Try using the client if you're using your browser or vice versa.

Thank you.