Are both Meridian devices running the latest fw, V4.00 for the 818v3, the 218 is updated via Sooloos or by running the Sooloos config program if you have never used Sooloos?
As a test connect the 818 or 218 and the Roon core directly to the SAME network switch then connect the switch to the router (do not use your router as a switch) does it work now?
This is a long shot, but are you running the very latest version of Roon? There is a fix in there for a bug with the Sooloos protocol losing connectivity to the end point. Other than that, I’d echo the previous comment about testing with an ethernet switch, rather than a router.
Do the devices have the standard configuration in them or have you put in a custom config?. They should work without issue straight out of the box provided your network is not restricting things.
www.cmbintegrations.com One step from the Presidency #1 Trinnov Altitude 32-8-16, 271, 8000XE, 7200SEHC, 7200.2, Seaton F18, JL Fathom F113, Revel w990's, Kef R7, ATI 528 Amp, JVC RS4100K, SI screen. Roon Nucleus+, Eno AG, Eno M link, 218. #2 Mark Levinson No5105, Ayre VX8 and CX8, Vandersteen Treo CT, Preamp varies. #3 Meridian Ellipse
Try turning off everything connected to the network - hifi gear, computers (especially the Roon core), switches, router. Then turn them on one at a time, but starting with the router, then any switches, then the Roon core, then last (really, last) any Meridian gear. Works every time here.