apcupsd is running as the master on IPFire. Therefore you have to set up the apcupsd program on your synology system as the slave. You need to have the UPSTYPE on synology set as net instead of usb so that it will be visible to the IPFire apcupsd and on the IPFire apcupsd.conf you need to have NETSERVER on.
Your screenshots look like your synology is running windows.I do not know if the windows version of apcupsd caqn run NETSERVER or not. You probably need to check the apcupsd manual on this.
The manual also covers linking two apcupsd systems, one running as master (the one with the UPS system) and the other as a slave, both having their power provided by the UPS.
As the master apcupsd is running on IPFire then any communications from it to other systems in green, blue or orange do not need any specific firewall rules.
I have a slave apcupsd running on a machine in my orange zone and communication from IPFire to it has no problems but the system in orange is running Linux so that is my experience.
Maybe other people have experience with a windows based slave system.