So I do this instead
wget https://vagrantcloud.com/bento/boxes/ubuntu-18.04/versions/202002.14.0/providers/virtualbox.box
file virtualbox.box
# virtualbox.box: gzip compressed data
mv virtualbox.box virtualbox.gz
gunzip virtualbox.gz
file virtualbox
# virtualbox: POSIX tar archive (GNU)
vagrant box add --name bento/ubuntu-18.04 --box-version v202002.14.0 virtualbox
vagrant box list
# bento/ubuntu-18.04 (virtualbox, 0)
If wget is too slow, you can use something like uGet, change the maximum coonection to 16, and rename the output (a UUID) to virtualbox.box
.