Background: I understand that Matrix was not included in Debian Trixie, and was happy to see that the FreedomBox developers developed a workaround in FreedomBox 25.11 to allow Matrix to be installed from Debian unstable.
Problem: I was not able to install Matrix even after the upgrade to FreedomBox 25.11. The plinth interface repeatedly stated that Matrix was not available for my version of FreedomBox. Logging into my FreedomBox’s terminal, the apt package manager did not see any packages for Matrix.
Solution: Apparently, my FreedomBox expected that apt sources for Debian unstable should be connected through Tor. I have not installed any of the Tor apps on my Freedombox, so instead, I installed the apt transport package for Tor (apt-transport-tor), updated my apt sources, and was able to successfully install Matrix through the plinth interface.