I've had Deco M5 system working well with an LTE modem/router for several years. 

Now I'm attempting to set it up with a new 5G mocem/router (Vodafone GigaCube 5G).

Network speed when connecting straight to the router is around 80-90 mbps. 

With main Deco wired to router, it's about 28 mpbs.

Turning off wifi function on router doesn't change this.

My previous modem/router gets lower speeds-- closer to the 28mpbs, ironically-- but doesn't have the drop in speed when connecting to Deco vs directly to router.

I'm afraid the problem is with the (lack of) settings on the modem/router. But is there any change to the Deco setup people would recommend?

PS Please be kind; this explanation has already maxxed out my tech skills.