I tried all the mentioned solutions, including a modem change and some technnical tweeks from the helpline. Eventually I bought a 3rd party router, Asus Zenwifi AX, and put the connect box into bridge mode - since then no problems. This is clearly a firmware/software problem on the side of the connect box.
Hope thats of some help.
I am also experiencing the same problems described in this thread:
I never had these problems with the old connect box. I am close to requesting a downgrade. Can anyone help?