I did the following steps:
$ sudo systemctl edit snapd.service
[Service] Environment=SNAPD_DEBUG=1 SNAPD_DEBUG_HTTP=7
$ sudo systemctl restart snapd.service
As stated on ‘Extremely slow snap downloads’ post, but when I tried again the
sudo snap install --devmode --beta anbox
It worked just fine.
Not sure what has fixed that, but now I have the app up and running. 
If I understood correctly, the steps on the other post were just for enabling a more detailed log. But after that the problem I was facing is gone.