RTL8125 2.5GbE Controller only using 1Gb/s, how can I use the full 2.5Gb/s? 20.04 LTS

I'm using a PCIe extension card with a RTL8125 2.5GbE Controller on it. I've installed the Realtek driver r8125-9.007.01 from here successfully, but when I run # lshw -class network I see capacity: 1Gbit/s.

  *-network
       description: Ethernet interface
       product: RTL8125 2.5GbE Controller
       vendor: Realtek Semiconductor Co., Ltd.
       physical id: 0
       bus info: pci@0000:28:00.0
       logical name: enp40s0
       version: 04
       serial: e0:e1:a9:57:80:94
       capacity: 1Gbit/s
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress msix vpd bus_master cap_list rom ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=r8125 driverversion=9.007.01-NAPI latency=0 link=no multicast=yes port=twisted pair
       resources: irq:38 ioport:d000(size=256) memory:fcb10000-fcb1ffff memory:fcb20000-fcb23fff memory:fcb00000-fcb0ffff

I've tried the following commands so far but they don't change anything:

• # ethtool -s enp40s0 autoneg on advertise 0x80000000002f
• # ethtool -s enp40s0 speed 2500 duplex full autoneg on

This ethernet port is connected to a Windows PC (that also has a 2.5GbE adapter) which shows 2.5Gb/s in the control panel correctly.

I tried copying a large file via samba and it works but only at 1Gb/s speeds. Any help is appreciated! It shouldn't be this hard to use a NIC at its advertised speed.

EDIT: After digging around some more sudo ethtool enp40s0 gives me:

sudo ethtool enp40s0
Settings for enp40s0:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
                                2500baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
                                2500baseT/Full
        Advertised pause frame use: No
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
                                             1000baseT/Full
                                             2500baseT/Full
        Link partner advertised pause frame use: No
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 2500Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: on
        MDI-X: Unknown
        Supports Wake-on: pumbg
        Wake-on: d
        Current message level: 0x00000033 (51)
                               drv probe ifdown ifup
        Link detected: yes

which looks good to me but the file transfer speed is still very slow for a 2.5Gb/s connection. Any alternative way to test the speed you can think of?

Edit2: Screenshot of iperf test Looks like the performance is only capped at 1Gb/s in one direction! Only when Ubuntu is the receiver I get slow speeds. If Ubuntu is the sender I get the full 2.5Gb/s. It can't be the cable at this point.

Try running insmod ./src/r8125.ko speed=SPEED_MODE duplex=DUPLEX_MODE autoneg=NWAY_OPTION while in the directory where you built the driver.