I thought Hmm would be in here to answer your questions but I haven't seen him, yet, so ...
Jagex's instructions for installing the client don't work in Ubuntu 19. In the past I posted instructions for older versions of Ubuntu that worked but weren't ideal and it looks like you might have tried using one of my posts to get to the point of missing libcurl.so.4. For Ubuntu 19 don't do that. Use the Flatpak or Hmm's Snap instead.
Neither are officially provided by Jagex but there haven't been any issues with using them and they do install the official clients. Each one fixes the issues with installing the client which are caused by Jagex not updating their DEB package for the NXT client.
Every supported Ubuntu flavour should support snap by default, barring Lubuntu 16.04 and 18.04, but even Lubuntu has it by default for 18.10+
as such, snap install rslauncher
should be all you need on basically any Ubuntu official flavour, though Snapcraft and Flathub have instructions for a lot of variety's in distros, so I'm hoping OP managed to figure it out from the websites themselves
I decided to have a little forum vacation the other day for a few weeks so I won't be around much, thanks for covering here though Ping