Almost certainly how your line is configured at the isp end, if your isp uses a Vlan ID setting of zero ( 0 ) no TP routers can currently use the 0 setting, the solution is to contact your isp and ask if they can assist quoting the Vlan ID setting as being the likely culprit.

Having read you again you don't need a modem with an ONT the ONT is the modem you need a router from the AX series but if that fails it'll be the above reason for it.