It starts with 139...
so if I get a router I can connect more devices?
Yes, you will probably need a router a switch is not enough. Set it on DHCP on the WAN port and let it create a local network on the other side. Whoever provides internet for you can detect what kind of device is at the end of the cable, but as long as you dont mess anything up I dont think they will block you. The best way to understand the setup is probably to ask them.
This is the correct advice. IP address of 139.x.x.x is an internet IP.
Get yourself a wifi router with extra ethernet (LAN) ports on it. Connect your current internet ethernet cable to the WAN port on the router. Connect your computers to the LAN ports. These routers usually come configured for DHCP on the WAN side, which is probably what you want. Check your computer and ensure that its adapter is set to DHCP, if thats the case then the router will have to be as well.
When your computers are connected to the LAN side your IP addresses will be from the router's DHCP server. So you'll have private class IPs. Either 172.16.x.x to 172.31.x.x or 192.168.x.x or 10.x.x.x