Help! Stuck at 100 Mb/s on Ethernet and 120 Mb/s on Wi-Fi—What Can I Do?

Hey everyone, I'm at my wits' end trying to figure out why my new PC build is limited to just 100 Mb/s on Ethernet and barely reaching 120 Mb/s on Wi-Fi. Every other device in my home manages significantly higher speeds—like 400-600+ on Wi-Fi and 650-700+ on Ethernet, using the same cables and modem ports. Here's what I've already tried: swapping multiple Cat-6 cables, connecting directly to my modem (Spectrum EN2251 DOCSIS 3.1), power-cycling the modem, updating the BIOS, clearing CMOS, and even reinstalling Windows 11 and Pop!_OS. I've gone through various Realtek drivers and adjusted settings like disabling Green Ethernet and forcing gigabit duplex, but nothing works. The specs of my build include an AMD Ryzen 9 9900X3D, MSI MAG X870E Tomahawk Wi-Fi, 64GB of G.Skill DDR5 RAM, and a Gigabyte RTX 5080. I'm starting to think there may be a hardware issue with the Realtek NIC not negotiating properly. Has anyone encountered similar problems or can suggest any other troubleshooting steps before I consider returning this motherboard?