RAX120 with M5 router passthrough Speed Issue.

Ok purchased a new RAX120 router. All firmware updated and everything now working well.

Had low speeds and after many hours with Netgear Techs, organised a replacement RAX120.

 The Issue I have is:

 I test my internet speed each day and since setting up the M5 with passthrough and stopping WIFI. My internet speed has plutoed at 200mbps. My usual internet speed can be up around 300 to 700mbps this can vary a lot being 5G service from Telstra. 

Not sure if the M5 is causing the problem or the RAX120.

The M5 is now only a modem and should be pushing internet speed through to RAX120.

Speeds acceptable but seems there is something stopping the speed from going above 200mbps.

Are there any setting that can be adjusted or checked to find the issue?

Appreciate the help