You can run a cable
next to your CAT cable. You just cant really use the twisted pairs inside the cat cable, unless you are using PoE devices, which deal with the interfierence.
The problem is that the
twisted pairs cause an electromagnetic field in the cable which is NOT part of the cable causing issues. Running low voltage, even high voltage with a SEPERATE cable next to it never caused me any issues. I even saw them cable tied to each other.
The problem only starts when you running Ethernet cable next to full phase cables, ie power to your house, or 2,3 phase cables for big machines. The electrical feedback caused by heavy machinery causes changes in the electromagnetic field on these cables, which affect Ethernet, WiFi, etc, and that is when it is strongly advised to run these cables far away from these power cables.
Your puny 0.6A modem wont cause any of this because it doesn't use mechanical engines which generate feedback, just simple linear voltage regulators and filters which help reduce this problem.
100 megabit or gigabit, always ran full speed.