XR500 is a bit better than the XR300 but it depends heavily on your usage. If you use a lot of wifi, XR500 is better, if not and you use all wired you will most likely not see much benefit.
The speed problem you have has to be a software of configuration issue as both WAN and LAN ports on that router is 1Gbit, or an issue with other parts of your setup.
For reference though, have a look in your settings if all is right. Go settings > monitoring > statistics and look at your LAN connection. It should say 1000M/Full. If it says 100M/Full then that's your issue. You might need to try another ethernet cable or redo the connections.
Some routers can be sensitive to the type of cable. I had one cable with a bit of damage once, on my ISP router it would drop to 10M/Full, on my XR500 it would drop to 100M/full. After cutting off 10cm and redoing the connector it went to 1000M/Full. These speeds get negotiated on signal strength as far as I am aware so that's why it can vary between devices.
I just noticed while looking that up that my PS4 on XR500 drops to 100M/Full in rest mode, as soon as it wakes up it goes to 1000M/Full. So it might be a software glitch but I would investigate the physical cable first. Cable needs to be Cat 5E or higher for 1Gbps.
The reason I suspect the cable is because usually when it tops out around 100 it's an issue with that. 100M needs 2 twisted pairs and 1000M needs all 4 twisted pairs. If there is an issue with one it will drop to 100M.