Using a VMG8825-T50 with CityFibre

I've just had a 100Mbps CityFibre connection installed by Zen Internet. It works fine with the supplied Fritz!Box router with 112M downlink and at least 300M uplink (!) Using the Zyxel, the uplink speed is only 46M. To check if it is an Ethernet problem I've tried a different cable, and connected it through a 1G Ethernet switch. Next, I've connected the Fritz to the ONT then connected the Zyxel to a LAN port on the Fritz, with the Zyxel WAN set to IPoE instead of PPPoE. This setup gives 300+M uplink speed, so the Zylinx is certainly capable of this speed. Any suggestions why the uplink speed is so low?