MinimServer and endpoint on different subnet

[Frodo]

I have 2 apartments that are connected via 2 Fritzboxes. Unfortunately, in my case I have to assign 2 subnets:

#1: 192.168.178.0 (MinimServer on Melco, Linn Exakt DSM, foobar2000 on iPhone) and
#2: 192.168.200.0 (AUDIOPHONICS I-Sabre DAC ES9028Q2M RPi 3)

Volumio is installed on the RPi and I was hoping that I can assign MinimServer via IP address like I did with foobar2000 no matter where I am. Unfortunately Volumio can only operate on the local network - at least how I understand the guide.

Anyone having a solution how I can bring the RPi to work using MinimServer?