Hello, I just registered here to share the solution I found for this particular router changing the MTU setting back to 576.
Apparently there's quite a few of us struggling with this issue and this model being a tad old means we don't get free support. (The bloody woman even suggested I buy a new 3COM router, "it's cheaper than paying for support!" Fool, like I'd want to buy ANY 3COM thing!!!) This solution may not work for everybody, but for those using a cable connection with static or dynamic addresses (with long lease times, like mine), it just might.
The problem is once you changed the MTU to 1500 and click the "apply" button, you check back and it's back to 576, no way to change it.
The solution, is to change "IP Allocation Mode"
on "Connection to ISP"
tab from "Dynamic IP address"
to "Static IP address"
, all settings spaces get filled automatically with the data it got from your ISP, now change MTU to 1500 (or whatever you wanted to, for that matter) Click the "apply" button, wait and presto! once you log back in, the MTU is changed and you can use your internet connection at full speed. Sweet, uh?
If your connection stops working due to IP change, set back to dynamic, apply go back to static and apply.
3COM hardware and support used to be so reliable, what happened?