Can not connect to Spectrum modem Hitron Model EN2251

I have now tired 7 different routers, updated firmware on routers, checked on updates to windows 10, tried new Cat5 cables from Hitron modem, rebooted modem and router each and every time a router was changed, looked at configuration pages on all the routers,  removed coaxial cable from modem re-installed and tighten, still can not get routers to work at the clients house.  All these routers work immediately / instantly on my Spectrum modem the Arris TM1602A, the routers all will run up to their maximum speed, and a triband linksys will speedtest at 437 Mps on the 5 Gig band.  Any suggestions.  Spectrum doesn't want to trade out the modem.