Modem is 100Mbit and I believe the WAN port is too. Properly installed Cat5e will support 100Mbit as an absolute minimum and should support 1Gbit.
You will not loose anything in that short run. Get a reasonable quality RJ45 socket and faceplate with matching backbox ... I would recommend the MKElectric range - or ADC Krone (if you can find a stockist who will deal with you). Standard Cat5e or Cat6 cable will terminate perfectly in those with a standard punchdown tool. Then a decent patch lead at each end - with flexible cable properly terminated into plugs (not a quick/easy job to start with).
I can easily run Modem >>>> Patch Lead >>>> Patch Panel >>>> 20+m Infrastructure Cat5e >>>> Socket >>>> Patch lead >>>> Hub
and if I was to increase that to: Modem >>>> Patch Lead >>>> Patch Panel >>>> 20+m Infrastructure Cat5e >>>> Socket >>>> Patch Lead >>>> Socket >>>> 20+m Infrastructure Cat5e >>>> Patch Panel >>>> Patch Lead >>>> Hub it would still work!
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