It could also just be the cable.. For gig to come up multiple things have to happen.. It could be the cable, it could be the negotiation between your media converter port and the sg3100 interface.
So yes connecting a PC that you know is gig and has shown gig connection before, with a cable you know has worked with gig is a valid test. I would do that test with the media convert to your PC and also the PC to sg3100 interfaces. Both its wan/lan interfaces and its switch ports.
Do you have another gig switch, be it smart/managed or even just dumb?
It could be something as stupid as if the media converter is managed that they have the interface set for set for 100 hard coded, etc. Or there is something off with the cable.
Also you sure your running the light correctly… What does pfsense web gui show for the connection speed? When you say
"connection is capped at 100 mbps up and down"
So your seeing low to mid 90's - 100mbps switch/port connection is not capable of actually doing 100.. If your speed test is showing you 100, and not say 95-97 tops say then that could point to a isp cap to your speed and not the actual interface being at 100.. The front page of pfsense should show you what speed the interface is at.
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