Nextcloud has its own very secure Chat- Client, that can be installed to your client.
You can find all Infos at: https://github.com/nextcloud/talk-desktop
Currently you need to manually download the flatpak from the Page and install it manually.
If you want to use scripted install for automated process, you may use this small nextcloud_talk_install.sh script::
#!/usr/bin/env bash
SCRIPTDIR=$(pwd)/$(dirname "$0")
echo Saving File to ${SCRIPTDIR}
wget -P ${SCRIPTDIR} --timestamping --continue https://github.com/nextcloud-releases/talk-desktop/releases/latest/download/Nextcloud.Talk-linux-x64.flatpak
sudo flatpak install -y ${SCRIPTDIR}/Nextcloud.Talk*.flatpak