According to boxmatrix, the FRITZ!Repeater 2400 uses the QCN5502 SoC.
Because the TP-Link Archer A9 v6.x has the same SoC, its chances of OpenWrt support will be similar. I suggest to refer to this post to get an idea what needs to be done. Are you planning to work on OpenWrt support for the FRITZ!Repeater 2400?
In case you just want to use a device without development, also consider the FRITZ!Repeater 1200 and 3000.
They both use the IPQ4019 SoC and are already supported by OpenWrt. Note that I do not own either of them, my recommendation is based on their specs and discussions in this forum.