What's the latency of your connection? You can get 4 Mbps throughput (which is not a lot by itself, BTW), but the latency may be too high for a video chat.
Yes, 4Mbps is not a lot, but since Skype shows something, Duo is effectively worse for me. I didn't measure the latency of my parents' connection. Will have to do it next time I'm there.
People can live with a few seconds of latency on GSM. Chat is mostly half-duplex.
It is usually lack of bandwidth or major packet loss that cause trouble.