When it installs, Ubuntu will, by default, install the GRUB 2 boot loader. When you boot, GRUB 2 will boot first, but it might or might not display a menu, depending on its settings and whether or not it's detected macOS. I haven't checked in quite a while, but the last time I did, Ubuntu's GRUB setup scripts did a poor job of detecting macOS. Thus, the system is probably booting to GRUB, which then goes straight to Ubuntu. If you're seeing either a GRUB menu or the rEFInd screen, then this is not right, and the below might not apply; please clarify what you see when the computer boots. If it's really showing no boot menu, though....
The easiest solution is to install rEFInd from within Ubuntu. You can do this by typing sudo apt install refind at a Bash prompt; that will install the version of rEFInd that comes with your Ubuntu distribution. To keep up-to-date with the very latest version, though, you can install the rEFInd PPA to get the latest version. After you install rEFInd in this way, it should appear when you reboot or start the computer "cold," and it should present options for both macOS and Ubuntu. The reason the installation you did from macOS isn't working is because GRUB 2 took over as the primary boot program once you installed Ubuntu. When using a non-standard boot manager like rEFInd, it must normally be installed after installing all the OSes on the computer.
Note that the rEFInd installed from Ubuntu will almost certainly replace the one installed from macOS -- it'll overwrite the files (keeping a backup of the original installation's configuration file and icons). Thus, you don't need to track down and uninstall the version you installed earlier.
