Hi,
We provide the FRITZ!Box 7530.
"up to 300 Mbps" - refers to the routers support of 35b supervectoring. This enables VDSL connections to achieve speeds of upto 300Mbps. Disclaimer - 35b supervectoring isn't offered on Openreach's VDSL network.
The 7530 handles 500Mbps and 900Mbps via the gigabit Ethernet WAN port without issue.
Worth remembering though that to achieve those speeds you should connect to the FRITZ!Box using an Ethernet cable. WiFi will give great speeds just not as fast as 500Mbps / 900Mbps.
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Andrew
ZeN Internet