ok tested it on a test vm running Q4OS 4.8.
On that vm I have a running FortiClient 7.0.3 now.
I achieved that this way:
1. download debian buster libappindicator1 and libindicator7 debs from packages.debian.org.
1.2. download forticlient deb
2. install all three with sudo dpkg -i with all three deb as parameters or download them all into the same dir and do sudo dpkg -i *.deb
3. it will complain about some missing deps so execute a sudo apt-get -f install afterwards to install the missing packages and finish the installation of the packages from step 2
worked fine on my test vm here. I can now start FortiClient on it.
The link to buster libappindicator1 is in my posting above. The the dependency list of that lib contains a link to libindicator7.
Hope you get it running too!
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