Linking from this similar topic:
Recommended values are 0x2a 0x21 as per Realtek specs, and there is a PR on Github. Once merged this should be fixed for all of us.
Thanks for providing the fix. For those who can’t use the dtb trick you can have a cronjob @reboot as a workaround:
@reboot echo 0x2a 0x21 > /sys/class/net/eth0/device/driver/fe300000.ethernet/rgmii_delayline