I don't know how to configure it using uci, but setting up something similar to this should work (I just tried tried it right now):
root@OpenWrt:~# ip link add link eth0 name eth0.9 type vlan id 9
root@OpenWrt:~# brctl addbr br-foo
root@OpenWrt:~# brctl addif br-foo eth0.9
root@OpenWrt:~# ip link set eth0.9 up
root@OpenWrt:~# ip link set br-foo up
root@OpenWrt:~# swconfig dev switch0 vlan 9 set ports '4t 5t'
root@OpenWrt:~# swconfig dev switch0 set apply 1
root@OpenWrt:~# mmcli -m 0 --simple-connect=apn=telenor.smart,ip-type=ipv4v6
root@OpenWrt:~# mmcli -b 1
--------------------------------
General | dbus path: /org/freedesktop/ModemManager1/Bearer/1
| type: default
--------------------------------
Status | connected: yes
| suspended: no
| interface: wwan0
| ip timeout: 20
--------------------------------
Properties | apn: telenor.smart
| roaming: allowed
| ip type: ipv4v6
--------------------------------
IPv4 configuration | method: static
| address: 10.195.138.65
| prefix: 30
| gateway: 10.195.138.66
| dns: 193.213.112.4, 130.67.15.198
| mtu: 1500
--------------------------------
IPv6 configuration | method: static
| address: 2a02:2121:28a:b893:bc8e:ed5d:115c:426a
| prefix: 64
| gateway: 2a02:2121:28a:b893:3cd1:8fe3:11c0:ef4c
| dns: 2001:4600:4:fff::52, 2001:4600:4:1fff::52
| mtu: 1540
root@OpenWrt:~# ip link set wwan0 up
root@OpenWrt:~# ip route add 10.195.138.64/30 dev br-foo
root@OpenWrt:~# ip route add 2a02:2121:28a:b893::/64 dev br-foo
root@OpenWrt:~# ip route add default dev wwan0 table 9
root@OpenWrt:~# ip -6 route add default dev wwan0 table 9
root@OpenWrt:~# ip rule add iif br-foo lookup 9 pref 10000
root@OpenWrt:~# ip -6 rule add iif br-foo lookup 9 pref 10000
root@OpenWrt:~# sysctl net.ipv4.conf.br-foo.proxy_arp=1
root@OpenWrt:~# sysctl net.ipv4.conf.br-foo.proxy_arp_pvlan=1
root@OpenWrt:~# iptables -I FORWARD -i br-foo -j ACCEPT
root@OpenWrt:~# iptables -I FORWARD -i wwan0 -j ACCEPT
root@OpenWrt:~# iptables -t nat -I POSTROUTING -o wwan0 -j ACCEPT
Note that you obviously should have saner firewall rules. And that this is missing the dhcp server config. Which really should be trivial, except that there doesn't seem to be a way to use dnsmasq for this since it makes too many assumptions. And you can of course don't have to configure vlan 9 for your inside interface. That was just for my test. The inside interface could be anything. But it will only support one IPv4 client.