From the SP31 User Guide:
Appendix B. Find Your Unit IP Address
If you connected the SP31 to your network but cannot open the management console by typing myminer.io in the
browser (see the SP31 Quick Start Guide), you can enter the unit IP address instead.
To find the IP address you can use one of the following methods:
Router Management Console
Log into your router management console and find the list of connected devices with their IP address. In the list of
connected devices, look for an entry beginning with “miner” and enter its IP address in the browser.
IP Scanner
Run an IP scanner to scan for devices connected to your network. You can download Angry IP Scanner from
http://angryip.org/download (or use any other IP scanner of your choice). In the list of devices, In the list of
connected devices, look for an entry beginning with “miner” and enter its IP address in the browse
If you're not having any luck with your router, I'd try the suggested IP scanner.
If you're still out of luck, I'd hard reset the miner and check again.
I'd also check if the miner is connecting to the wrong address range (e.g. 192.168.0.X, but your router is set to take addresses on 192.168.1.X).