Hmmm I am testing next to the router with a dongle. Looks like I have a defective unit. I have registered it with ZyXEL and as far as I know it is still under warranty, haven't heard back from the seller yet. Is that on 5ghz as with 2.4ghz I only get 144.
Further experiments and checking on windows dongle status shows the 5g connecting at 144mbps and the 2.4g at 300. I would have expected 300 on the 5g unless it is bandwidth limited.