Can't change MTU on eth0 (WAN)

[ugvu]

When I'm using command

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ip link set mtu 1280 dev eth0

it returns

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rk_gmac-dwmac ff540000.ethernet eth0: must be stopped to change its MTU

.
So I make the link down to use the same command but it returns

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SIOCSIFMTU: Invalid argument

Does anyone have the same problem?