100m of cat5e between modem and router should have no impact on performance.
The length limit of 100m for ethernet comes only when on solid core cable.
The length is stated to be 30m when using flexible patch cable. I don't know of any practical tests that have tried to go further.
THe other important desctinction is how to terminate solid core and multi core.
ONLY Solid Core shuold be used in the IDC connectors present on patch sockets
ONLY Multi core should be used with RJXX plugs.
So > Multicore for plugs, solid core for sockets
Ever tried to put multicore into Insulation displacement connectors like those on the back of a phone socket? Doesn't work out too well...
BT Infinity #2 - 4.4MBps on P2P ¦ Full 40/10 Sync ¦ 81/20 Attainable ¦ Benchmarked 22ms Ping ¦ 2ms Jitter ¦ 37.3Mbit Down ¦ 9.2Mbit Up
BT / Openreach - You've finally done well. Very well indeed.