Hi,
I ran into the exact same problem and I am really grateful for your posted solution.
However, as a newbie, I am a bit unsure how to execute step 2: "Create a new network (i.E. my-net) in bridge mode and put your letsencrypt and nextcloud containers both in here"
Just to make sure: Part 1 of that step is executed through the docker UI, in the networks tab, right? I get that part. And the two containers I add by choosing the relevant network in the settings of the relevant containers?
I think I might have it right, just trying to verify before I start messing something up. 
Thanks in advance!