Not sure if this helps @amosbatto, but I’ve just tested abbreviated version (easier one) of above posted install command: sudo apt update sudo apt install mono-complete(5.18.0.240+dfsg-3_amd64.deb) Do you want to continue? [Y/n]
Therefore I doubt that anything related to current mono-complete package is missing within PureOS repo, yet: sudo apt dist-upgrade --fix-missing
P.S. Another, if someone insists, approach might be on how to use http://download.mono-project.com repository with PureOS, but I believe (somehow) this is out of the subject here.
# deb http://deb.debian.org/debian jessie main
# deb-src http://deb.debian.org/debian jessie main
deb https://repo.pureos.net/pureos/ amber main
deb https://repo.pureos.net/pureos/ amber-security main
deb https://repo.pureos.net/pureos/ amber-updates main