I have symmetrical 1.0 Gbit from my ISP. I have my router connected directly to the cable modem and then connected to a smart switch to take care of VLAN tags. There is a dumb access point also connected to the smart switch. I see that the switch has a port for 2.5 GbE SFP connections but I don't understand what I would use it for.
SFP usually is for a fiber optic connection, but not always.
I cannot get standard cat6 patch cable to even fit in the connector. It is not RJ45.
if it's just a square hole, you need to buy a ethernet SFP module.
Correct - that was already explained above. I even mentioned fiber optic cables, so I'm lost at why you tried RJ-45 CAT6 copper?
I'm not sure why you're under the impression it's RJ-45, I even provided a Wikipedia link - feel free to take a look.