Finding which device on my network has a certain IP

Easiest from a phone: install any network scanner app, run it on the same wifi and it lists every device with IP, manufacturer and often the device name. Takes about ten seconds and you don't need the router password.

Your router knows. Log into its admin page and find the connected devices or DHCP client list, it shows every device with its IP, name and MAC address. From a Windows PC you can also try ping -a followed by the IP in Command Prompt, which sometimes returns the device name. Then run arp -a to get the MAC address for that IP. The first half of a MAC identifies the manufacturer, so a lookup on it often tells you if it's a printer, a TV or someone's phone.