Some routers will call this address reservation, it will be in the DHCP section, you will also need an IP, i like to use the .1xx range for reserved addresses because it doesnt overlap the likely number of DHCP addresses assigned, not that it matters if you have it reserved. Once you pull that section up, it should have a list of connected devices, most will have a name or manufacturer associated with the 'MAC' which will look like xx:xx:xx:xx:xx:xx, and is how the router identifies which device is which when handing out ips.
This will make sure that when your apext connects to your router, it will always have the same ip. Note, this is the private side of your network, and any ip you assign there is not going to be routable outside of your network. The 'static ip' that some are talking about is assigned from your service privider. To get that ip statically assigned to you from them it is usually an additional monthly fee, and then for you apex to be able to use it, you would want to port forward it. If you bridge it, all the other devices attached to your network will probably stop working. Bridging just takes the 'router' out of your router and it will just pass whatever straight through. Home devices aren't usually set up to accommodate that though and i highly doubt that is what you need.
I would think more you need to hard cable it and it is more an issue with the wifi connection, although if you really wanted to you could reserve it an ip first for fun, but that is unlikely your issue.