RealtekRTL8111 shows gigabit but won't exceed 100Mbps

[mook2420]

RealtekRTL8111.kext
Version 2.2.1 (2016-03-12) requires macOS 10.11 or newer
version 2.2.2 (2018-01-21) requires macOS 10.12 or newer
version 2.3.0 (2020-08-14) requires macOS 10.14 or newer
version 2.4.0d4 (2020-10-10) requires macOS 10.14 or newer

Ahh, dang that does make sense why 2.3.0 never worked for me on 10.13.5/6 at all, haha. Thank you for that bit of information.

Final update I hope: I purchased and installed an Intel I210 PCIe NIC and Bob's your uncle. It loaded the AppleIntelI210Ethernet.kext and I removed RealtekRTL8111.kext. Speeds are back to gigabit, huzzah!