Dual WAN with PPOE and Ethernet 4G ZTE Router

Hello !
I am very new to openwrt...
I have a Linksys WRT3200ACM router with OpenWrt 21.02.1.
I have a WAN ppoe connection and a secondary ZTE MF255V router which I want to be the secondary backup connection.

I used this video to enable DUAL WAN :

Can you help me to configure my router, please !

Thank you very much !

you watched the video, what's the problem ?

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The problem is : still I can't make it work...

My primary Linksys WRT3200ACM is connected to the internet through WAN interface via PPPoe protocol and it is working fine. IP address is

My secondary ZTE MF255V router is already configured with a static IP address and it is working fine if I connect a wired laptop to it.

I configured the second the secondary wan interface WANB as described in the video but I don't know what protocol to choose for the connection ( it is not ppoe). As I said, the secondary router is already connected to the internet through a GSM mobile card and it is working...

Which protocol should I choose from this list DHCP Client, DHCPv6 Client, PPP, PPPoe, Static Address and Unmanaged.

I tried to switch between those protocols but no luck yet...

Only when I choose Static Address the WANB interface is green but still no internet on LAN if I disconnect the primary WAN cable...

Thank You !