Thanks to this answer in another topic I managed to finish the setup and now use happily my new Archer C7 V2 router with a bridged LAN port to the WAN.
Using the clarifications in the post I managed to configure the router using only the web UI
Those are the current settings of the switch section and the interfaces section regarding the setup I needed.
config interface 'loopback'
option ifname 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'fd48:48c1:61fb::/48'
config interface 'lan'
option type 'bridge'
option ifname 'eth1'
option proto 'static'
option ipaddr '192.168.24.1'
option netmask '255.255.255.0'
option ip6assign '60'
config interface 'wan'
option type 'bridge'
option _orig_ifname 'eth0'
option _orig_bridge 'true'
option ifname 'eth0 eth1.3'
option proto 'static'
option ipaddr 'STATIC-IP-ADDRESS-HERE'
option netmask '255.255.255.0'
option gateway 'GATEWAY-IP-ADDRESS-HERE'
option broadcast 'BROADCAST-ADDRESS-HERE'
option dns 'DNS-SERVERS-ADDRESSES-HERE'
config switch
option name 'switch0'
option reset '1'
option enable_vlan '1'
config switch_vlan
option device 'switch0'
option vlan '1'
option vid '1'
option ports '0 2 3 4'
config switch_vlan
option device 'switch0'
option vlan '2'
option ports '1 6'
option vid '2'
config switch_vlan
option device 'switch0'
option vlan '3'
option vid '3'
option ports '0t 5'
config interface 'vlan3'
option proto 'static'
option ifname 'eth1.3'
Hope this helps someone else eventually.
EDIT:
Added /etc/config/network for further clarification.