If they had a HH5 I would have recommended using the modem as this doesn't reboot regularly like the homehub does, so generally DLM behaves a lot better... With the smarthub it's not possible to use a modem unless you have Infinity 3 and 4 and you get a slightly different variant for FTTH services.
Typo was meant to be hh4 and not6. But openreach modem will work with hh3 and 4 as designed this way and hh5 if required. The line management or slow down does not happen if the router reboots as the modem is still connected. The older bbu batt back up kit was great for this as well.