@Nottes
Hi,
Cat5 is officially specified to up to 100 Mbps.
However, if the cable is not very long and the manufacturer made them in good quality, then there is still a good chance that the BE450 and the RE900XD may establish a 1000 Mbps connection via that Cat5 cable. But nobody can guarantee this in advance.
Before you start to build your network I suggest you power up the RE900XD and check what firmware version it comes with out of the box. There is a chance that the factory firmware may be to old to support EasyMesh Ethernet backhaul or even EasyMesh as a whole. In this case you would need to first update the RE900XD to the latest firmware version.
The easiest way to establish an EasyMesh network would be by using the WPS method. Press the "WPS" button on the BE450 and right after that press the "WPS" button on the RE900XD.
Once the EasyMesh is working (check within the router's GUI) you can bring the RE900XD to its destined location and plug one end of the Cat5 cable into the RE900XD and the other and into the BE450.