As I said before a normal network cable has four pairs of wires, these four pairs can run up to 1Gbps as a single cable.
Now you can split these four pairs with the splitters so each link now only has two pairs, this can run at 100Mbps, and seeing as the modem only has 100Mbps ports this OK.
Presuming your 30m Ethernet cable has plugs each end you will need two splitters and four short patch cables, plug the splitters into either end of your 30m cable, plug the patch cables between the splitters and two ports on the router, the other end of the 30 meter cable plug into the osplitter and then LAN 1 & LAN 2 on the modem.