Linksys wrt54g speed trouble

Hi, I am new here and I have a little problem with my Linksys WRT54G router. I can connect to internet but I can't connect to the File server my ISP is offering to me, at least not at the speed I should get. I should be able to connect to their server with 54 Mbps (as this is 54Mbps router not 100 Mbps) but I can only connect with the same speed as my internet is going. I had the wired Linksys befsr41 and since I had trouble with it I changed it with the wireless one. In the meantime I had been connected to internet directly to my lan card on the PC and I had the full 100 Mbps speed. Now My ISP is telling me that this router of mine (WRT54G) can't be set to work with the range of IP addresses they have on their server and that's why I can't connect to full speed. Is it true? Thay said that with another brand, like ASUS (wired router not a wireless one) I won't have that problem. Is there another solution to make my router work? Thay told me to try putting a switch before the router and connect it to second lan adapter but I couldn't make it work. Can anyone help me? Thanks.