Rockylinux8.4 - Network Interface file (ifcfg-enp3s0) config MTU=1000 ,Restart network interface ,ifconfig enp3s0,show mtu=1280 ,not show 1000.
Try
nmcli connection modify enp3s0 802-3-ethernet.mtu 1000
systemctl restart NetworkManager
What do these show:
nmcli con show enp3s0 | grep -i mtu
ip link show enp3s0
(The ‘ifconfig’ got an improved replacement ‘ip’ – of iproute2 package – over two decades ago. That probably does not matter here.)
(In RHEL 9 NetworkManager will no longer write/use legacy ifcfg-* files, at least not by default.)
PS. “Rockylinux8.4”? “8.4”? Rocky Linux is currently based on sources of RHEL 8.6 – the 8.[45] are no longer supported. NetworkManager appears to have been rebased in both 8.5 and 8.6; new features and fixes.