As long as FortiClient didn't support IPSec on Linux I switched to Strongswan coming with almost every distro in the standard repos. Thus that has no gui and you have to configure everything manually (which I don't mind) it works fine with our FGT at Work.
There is a plugin for strongswan and ubutu's networkmanager but afair it did not provide all settings i needed to make the vpn work.
Meanwhile btw the FortiClient for Linux does support IPSec. Correction: it still does only support sslvpn in linux! Fortinet just have to fix their dependencies for they can no longer be met in some current distros anymore. It still does install fine in Ubuntu 22.04.
However one can workaround those dependency issues by using some packages from debian buster to meet them.
"It is a mistake to think you can solve any major problems just with potatoes." - Douglas Adams