Diagnosis
Your computer's network configuration has likely drifted, or the router's DHCP server is glitched. You're not getting a proper local IP address from the router.
How to Fix It
On your Windows PC, open Command Prompt as admin and type ipconfig /release then ipconfig /renew. On a Mac, go to System Settings > Network, click your connection, and toggle it off/on. This forces a fresh request for an IP address. Then try 192.168.50.1 again.
If That Didn't Work
You'll need a direct wired connection. Plug a PC into one of the router's LAN ports with an Ethernet cable. Manually set your PC's IP address to 192.168.50.10 with a subnet of 255.255.255.0 and gateway of 192.168.50.1. You should now be able to access the interface at 192.168.50.1.
Heads Up
If you changed the router's IP address range in the past (like to 192.168.1.x) and forgot, you'll need to do a factory reset. Hold the reset button on the back for about 10 seconds until the power LED blinks. This wipes all settings.