https://wikidevi.com/wiki/Linksys_WRT54G_v5.0
It's a 2005 device, that didn't ship with Linux. The wireless protocols are wildly out of date (802.11n was 2009) and can't be updated. With 2 MB of flash and 8 MB of RAM, it's pretty much unsupportable.
There are many suggestions and opinions of inexpensive (US$20 or so) devices that can make you much more secure, faster (in 2005 a 56k modem was pretty fast, not the 1,000,000 kbps lines available at consumer-friendly prices), and a lot easier to configure in the long thread