Unable to open .acsm

Using the latest version of ADE, I'm trying to open an old .acsm file (from 2013). Running MacOS Big Sur 11.2.1

I'm getting the following error message:

Connection Error Detected Digital Editions. Could not connect to the fulfillment server. Please make sure you are connected to the internet.

I am definitely connected to the internet. There's no firewall or anything. I've looked around the forum here and found no solution. Someone found that using ADE 3.0 worked for them, but 3.0 does not work on Big Sur apparently. Someone else found that ADE 4.0 works, but I cannot find a download for that.

Please advise ASAP. I just spent like 3 hours on your customer service chat being transferred to 5 different people only to be told to post here. Thank you.