I’m on 5G here and have no problems so I don’t think that is it. If you are doing the downloads through your browser I’ve found Firefox is pretty unreliable and fails to download often. Usually hitting retry and waiting a few second fixes it though so I’m not sure if that is what is happening to you. I instead use the link grabber feature through Jdownloader and download through there. Stremio performance is flawless for me, except when Debris itself or the add-on is down.
Edit: Though I’ve never heard of an ISP doing this I presume they would be able to throttle any traffic with you connecting to a debrief server, I reckon that is highly unlikely however.