So I just hooked the AX5400 to the isp router thats in modem mode but my speeds through ethernet have dropped from 950Mbps to 700-750 and iv only got once device connected too.
Also tried differnt ethernet cables but made no difference so not sure what else to do.
How are you connected, via Ethernet or WiFi?
I cant find any benchmarking data on this router. Basically its like this, Gigabit ports, dont mean Gigabit Internet capable. You need Gigabit ports to get more than 100 Mbps. But if the router cant do NAT as Gigabit speeds, then well you wont get Gigabit speeds.
I would assume a triple core device like that could do Gigabit, but I cant find any benchmarking data. This is one of the reasons people with Gigabit choose to build a router via PFsense rather than buy an off the shelf one, because a majority of routers cant support Gigabit internet properly. There is a possibility that you have something set wrong, or some feature on the router is eating too many CPU cycles.