The engineer recommended it the last time as he says it gives a more stable connection
No, all it does is to hide your real connection speed, unless you have an unlocked modem where you can read the sync speed.
Have you tried connecting directly?
The last recorded connection speed is shown here, which seems to indicate that the modem is not connecting at the correct speed.
Is there any noise on your phone calls? Dial 17070 and select option 2, there should be no noise between the announcements.
Perhaps you have an issue with your internal phone wiring?
It would be worth seeing what the HH5 connected directly gives.
Can you post hub stats. Troubleshooting then helpdesk