obviously popping in and out not stable
Nope. Wireless and wired have different MAC addresses and will therefore have different IP addresses if you switch between them.
Connect the xbox LAN cable, disable wifi on the xbox (to ensure that it is forced to use the wire), reboot the xbox (to ensure that it does not use any wifi settings). What does the xbox now show for IP address & Gateway?
I have a tplink switch which i connect to port 3or 4 on the router via cable the I connect an ethernet cable from the xbox to the switch then works fine, why?
That probably means that cable you are using for the xbox just does not want to sit right in ports 3 or 4, or is faulty in some other way. If ports 3 & 4 work when a switch is plugged in they should work when any other device is plugged into them.
As for the IPv4 vs IPv6, just ignore that.
Hi - on three previous occasions, Optus must have remotely diagnosed your modem and considered then all faulty and the Tech verified the same on the fourth replacement. You must have Factory Reset the current modem somewhere through troubleshooting to clear out any code glitches and I would expect it would be at Firmware suffix Ver 105 or 109 by now. So maybe when the XBox is directly connected to the modem with Ethernet, the modem thinks it gets overloaded and limits the Ethernet ports to only 2 * instead of 4, not caring which 2 ports are utilised. It's hard to accept that 4 units could have the same disfunction, but you never know with electronics - the chances of the factory batch production serial numbers on all your cardboard boxes being sequential are fairly remote.
* not sure if the TP Switch connection was one of the two active ports or it linked OK to port No 3 when your PC and the Fetch were in port 1 and 2?
The F@ST5366TN has the one of the largest power supplies for modems I have seen, 12V DC and 3.5 Amps which should be more than capable of handling fully equipped operations - have you been using the original power supply from the first modem on successive replacements which may have an issue, or have you used a new power supply which accompanied each successive replacement modem? A year ago, after having years of connection issues with an external hard drive, I would get a green LED and hear the disc spin and then stop, sometimes I would see contents, other times nothing, so I gave up using it until I replaced its power supply, using one from a spare Netgear router, and it has not failed since.
I agree with you , you shouldn't have to compromise - things should just work but, if no improvement, the workaround configuration using TP Switch may be a viable interim solution until Optus modem No 5 arrives. It is also possible to use the F@ST 5366TN as a secondary modem delegating it to be a Wi-Fi and in your case, a limited 2 port LAN Access Point with its VoIP service intact and use an alternative Primary Router which supports the XBox Ethernet connection. There are YesCrowd forum posts on these arrangements.