I have had top of the line routers where all they had was 100baseT or only 1 gigabit connection. Next would be to make sure the jack you are plugging into is actually gigabit. Since 2007 iMacs have been gigabit so being an older iMac has nothing to do with it. If so then maybe take a good look at your jack and make sure none of the pins are out of alignment or pushed down where they don't make contact. While under test wiggle the cable about at the connectors and verify all 8 wires stay connected. I would recommend a cat 5 cable tester and check the cable to make sure there are no intermittently loose connections. The jacks are a little different from manufacturer to manufacturer. If the cable you are using worked on a windows machine it really isn't eliminating the jack or the cable. I have seen brand new cables have less than shall we say great connectors. The difference between 100 baseT and 1000baseT is pins 4&5 and 7&8. I am really upset now, what shall I do, is there something wrong with my setups? Is it a connection that it is not Ethernet setup but PPPOE? I called my ISP they came with a windows OS based laptop, plugged in the ethernet cable and it basically flew with 700Mbps at least, so the cable is good (CAt5e), my ISP is providing 1000 Mpbs on Win laptops its super working, super fast. Also I would like to use internet via wi-fi router (gigbait router), but will the Airport Extreme card (Supported PHY Modes: 802.11 a/b/g/n) handle speed above 100? The cable is brand new from ISP guy, cat5(e). I changed my ethernet manually to 1000baseT, but then said cable is not found or not good. My ISP is giving me 1000 G net, but on the speedtest it doesn't go above 100. I read other posts about this problem/topic, but I couldn't find a solution. I am new here but with a same old problem.

2009 mac ethernet