If you want full connectivity on the second Rpi you need to run the routed service in the RPi with both connection to transform it in router. Then every computer in the network need to know the route to go to the other subnet.
There is other possibility if you not need full connectivity, for example using ssh tunnel to the first Rpi that end to the second. Or for http run a proxy on the Rpi with both connection.
This is not exactly your configuration but this can help to understand the router concept:
Forget ST4 if the mount driver support pulse guiding, this is the case for INDI Eqmod.
The interest of ST4 is only for non-computerized mount where it is the only option. For example if you use the HEQ5 only with the handpad without computer connection.
I am not sure what you see in Astroberry but here are two standalone control panel client I use.
DCD is a Python client:
My IndiStarter software include a client you can use even if starting the Indi server another way. After installation look for INDIgui in the system menu.
At the moment the fact is that despite the old FITS usage and the recommendation in the WCS document many file from camera are top-down.
This the case at least for the two I can test, indi_gphoto that return the FITS rows in the same order than libraw and for indi_atik from the Atik SDK. This is probably also the case when using other manufacturer SDK.
But it is not necessary to change the way the data are written to the file because this can affect the performance. So probably no alignment code need to be changed.
The idea with adding this new ROWORDER keyword is to clarify the way the processing software treat the image specifically with bayer matrix. All of this with minimal change in the capture software.
So if a driver know the camera return data in top down order it just add ROWORDER = 'TOP-DOWN' in the header, ROWORDER = 'BOTTOM-UP' in the other case.
Redhat server is really not intended for this use, as you remarked many required packages are not available on this system.
A good start is to install EPEL which add many packages from Fedora to Redhat server:
Then for everything else that is missing you need to install from source. This can be a big work to solve all the dependencies and even more work to maintain.
The interest of Astroberry is to not have to do this kind of operation, but if you really need it the phd2 build instruction are here:
The yellow background for the top message is probably because of an incomplete dark theme because the color is not fixed in code.
For me using the Gnome standard theme Adwaita-dark the message background is dark gray. This theme for Gtk2 is normally available in the package gnome-themes-extra-data.