I have been able to run a script at startup that enables 2.5g on the Intel X550. Here's how :

Create the script that will run at startup

nano /usr/local/bin/x550.sh

Content of x550.sh :

#!/usr/bin/bash

sleep 1m
ethtool -s eth1 advertise 0x1800000001028
echo `date +%F" "%T` "Startup script worked" >> /root/x550.log

Make the script executable :

chmod +x /usr/local/bin/x550.sh

Create the service that runs the script at startup

nano /etc/systemd/system/x550.service

Content of x550.service :

[Unit]

Description=Runs /usr/local/bin/x550.sh

[Service]

ExecStart=/usr/local/bin/x550.sh

[Install]

WantedBy=multi-user.target

Enable the service

systemctl enable x550

Reboot and run this command to see if the link speed is 2.5g

ethtool ethX (replace X with yours)

It should show this result :

Supported ports: [ TP ]
       Supported link modes:   100baseT/Full
                               1000baseT/Full
                               10000baseT/Full
                               2500baseT/Full
                               5000baseT/Full
       Supported pause frame use: Symmetric
       Supports auto-negotiation: Yes
       Supported FEC modes: Not reported
       Advertised link modes:  100baseT/Full
                               1000baseT/Full
                               10000baseT/Full
                               2500baseT/Full
                               5000baseT/Full
       Advertised pause frame use: Symmetric
       Advertised auto-negotiation: Yes
       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: d
       Wake-on: d
       Current message level: 0x00000007 (7)
                              drv probe link
       Link detected: yes

You can check the log file of your script located at /root/x550.log

I used this article to create everything :
https://www.redhat.com/sysadmin/replacing-rclocal-systemd