Remote player not recognised

I am trying to run Roon 1.18 in is latest version from an iMac connected by ethernet cable to a DrayTek Vigor 2762ac router which is itself also connected by similar cable to an Arcam DAC and hifi system. If I do that I cannot see any connection tot he DAC either in the Mac’s sound output options - clicking on the loudspeaker symbol in the top bar - or as an audio zone in Roon. On the other hand if I connect to the DAC directly by ethernet cable from the iMac it will show the connection both in the Mac’s sound output options and, eventually, as an audio zone in Roon. (I say eventually because it will often only show like that if I load Roon after I have switched on the DAC and hifi.)
Is this the second one the only way to connect the system? I thought it should pass through the router to the hifi without difficulty and the hifi be recognised. I would prefer to keep the iMac connected by cable to the router if possible so as to avoid extra load on the household wifi system and increase speed of download. What am I doing wrong? Any help gratefully received.