I have an FS.com S3900-24T4S switch (24 1Gbps RJ45 ports, 4 10Gbps SFP+ ports). According to its manual the SFP+ ports are only 10Gbps -- they cannot negotiate a slower speed. I put an FS 10Gbps SFP+/RJ45 transceiver in one of the ports and verified it cannot detect a 2.5Gpbs device connected plugged into the transceiver's RJ45 port. However, while browsing Amazon, I saw a handful of 10Gbps SFP+/RJ45 transceivers that say they can work with 5G/2.5G/1G/100M Ethernet devices.

Between the SPF+ port and the RJ45 transceiver, which piece is responsible for negotiating speed with Ethernet devices? The SPF+ port or the transceiver module? Or do they both have to support speed negotiation?

Edit to add: The 2.5Gbps device is a home WIFI access point. It works fine on the switch's 1Gbps ports. It would be nice to make sure it has the full throughput.

Some reading suggests that for RJ45 it's the SFP module that does the negotiation. Buy one from Amazon and return it if it doesn't work.

You can buy a copper SFP+. The copper SFP+ can do speed auto-negotiation. You need to check whether the copper SFP+ support 2.5G.

I have the exact same setup. Were you able to get 2.5gbe working through the SFP+ port?