Wireless set up and mtu

I have installed openwrt on an old archer c7 and I am having a bit of confusion on what to use as Network option when setting up the WIFI radios.

In the interface Configuration I have three Network Options (lan, wan, wan6).
It defaulted to lan.
Is this the correct one?

my Internet provider told me that the MTU size should be 1400.
In Devices under Network/Interfaces, there's a list:
Device Type MTU
Type: Bridge Bridge device 1500
Device: br-lan
Type: Ethernet Switch Network device 1500
Device: eth0
Type: Switch VLAN VLAN (802.1q) 1500
Device: eth0.2
Type: Ethernet Switch Network device 1500
Device: eth1
Type: Switch VLAN VLAN (802.1q) 1500
Device: eth1.1
Type: Wireless Adapter Network device 1500
Device: wlan0
Type: Wireless Adapter Network device 1500
Device: wlan1

Which Device is the one that I should change the mtu? is it Br-Lan?

It's the default, yes.

(And I wonder why you think LAN is incorrect - given WAN is the interface connected to your ISP.)

Then you would set this. I assume you meant "interface" and not "device" (which is the OpenWrt in question). It should be set on all interfaces to prevent fragmentation of packets destined for your ISP (i.e. the Internet).

(And I wonder why you didn't think to ask about WAN - which is the interface connected to your ISP.)

Thank you for your help.
Its very confusing to me the nomenclature that openwrt uses.

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OK, you were using OpenWrt terms, cool.

Then network device and interface are pretty much interchangeable terms in this case. The web GUI is straightforward on how to make the MTU setting.