I have the same problem with the NBN to. Had it on ADSL and now NBN. Using TG799 on NBN and it still happens. Checked everything everytime but stilll re occurs. Green lights stay on on Moden but no internet. Rest modem works again. Changing wifi chanels doesn't work either.
My advice is keep a log of all faults, The matter should be escalated if you have 2 or more documented instances of no service. I have had this problem for over 2 years on and off. Good luck.