here you have a last nightbuild offline installer tested with opensim 9.1
here you go....
extracted with folder sudo -i root and you password
tar -zxvf archivename.tar.gz
open archive folder /root or where you have uncompressed...
dpkg -i *.deb and wait
then done go in a fresh terminal ,mono --version and as all are ok see you last mono 5.13 nightbuild 7 -3-2018