I used to use an N300 and had the same issue, but use an N510 IP PRO now.

I remember trying all sorts of settings - not sure if I did resolve it - but in Settings > Telephony > Connections > Automatic check for profile updates - I set this to NO

Not sure if this did anything as a while ago - and I know it doesn't sound directly related to SIP registration.

In reply to a post by HAli24:

I have heard that if I change transport type to TCP it will help for CGNAT.

The VoIP provider needs to support TCP as well for this to work

I have finally got static IP just hoping this solves the problem so far its seems good had an outage but was only for like 15 minutes. Won't know for sure until an actual outage pretty long or something like that.

I had the same issue with mine around 10 years ago and I was using Google DNS. Switched to another DNS and all ok.

Never had an issue after that. However n300 only supports ipv4 so you're stuck if behind CGNAT which Octaplus use. I have just recently moved to using a Fritz!box that Zen supplied as my SIP gateway ( I have a separate Asus router for the connection) the Fritz!box supports SIP over IPV6. I registered my existing handsets to the Fritz, almost seemless! This information might be useful to others behind CGNAT although you're paying for a static IPV4 so your problem should go away now.

Now got new problem. Started today the n300 showes its registered but I get no connection when calling sometimes and also it says phone is busy when calling someone. I restarted and seems fix problem but what is happening can anyone help.

Please let me know if you ever resolved your issues using the N300 over Octaplus.
Did Octaplus ever fix the dropouts, or help you in any other way?

Octaplus offer cheap broadband from City Fibre, but if they dont support VOIP I'll have to give it a miss.