There are several computers with a PowerBook G3 name. Which exact model do you have?
https://everymac.com/systems/apple/powerbook_g3/index-powerbook-g3.html
Operating system version?
Using Ethernet with a DHCP setting is normal for a connection to one of the LAN ports of a router.
In what way does it not work?
Are you using a web browser (which)? Please note that an old web browser has difficulties with all modern (secure, https) web pages, so any practical web use will be very limited.
You should however be able to connect to one of the few plain http sites on the Internet (for instance, knubbelmac.de).
May or may not work be possible to access an internal setup page of a router. Which T-Mobile router model?