I have installed flatpak Octave 5.2 on two Mint 19.3 PCs following the instructions on flathub (
https://flathub.org/apps ). As Mint 19.3 is based on Ubuntu 18.4 you should not have any problems. I think that the flatpak infrastructure is already installed in the Mint distribution.
https://itsfoss.com/flatpak-guide/ explains how to check if it is installed in your distribution and is it is not how to install it. It also gives details of the general use of flatpak.
John C Frain, Ph.D.
Economics Department 3 Aranleigh Park
Trinity College Dublin Rathfarnham
College Green Dublin 14
Dublin 2 Ireland
Ireland