@devinplombier , got it; makes total sense. Thank you.
I have returned the iFi silent power unit and for now, i am going replace it with a duplicate the double StarTech/Finisar optical “moat” with the OM4 Corning glass in between the switch and M33 (i.e, the last ethernet segment to use your nice phrase). I may just leave the connection between the modem & router, alone for now. (I put aslightly better power source in to the router a couple weeks ago; maybe that is sufficient, given the “moats”.)