Let me sent my configuration in case it can be usefull 
The wan device
Wan bridge (I had 2 wan ports but I only listed the current connected one eth2, for you probably eth0 I would suggest if theres no bridge u keep it that way
):
uci show network.@device[1]
network.cfg050f15=device
network.cfg050f15.name='br-wan'
network.cfg050f15.type='bridge'
network.cfg050f15.macaddr='00:00:00:00:00:00' <- should be the mac address of box12 modem.
network.cfg050f15.ipv6='0'
network.cfg050f15.bridge_empty='1'
network.cfg050f15.ports='eth2'
Wan device (note: I dont see a port listed so I dont think ive used this at all but I did had my mac address set to box12 mac):
uci show network.@device[2]
network.cfg140f15=device
network.cfg140f15.name='wan'
network.cfg140f15.macaddr='00:00:00:00:00:00' <- should be the mac address of box12 modem.
network.cfg140f15.ipv6='0'
The pppoe wan interface
Wan interface:
uci show network.@interface[7]
network.wan=interface
network.wan.proto='pppoe'
network.wan.username='00-00-00-00-00-00@internet' <- should be same mac address as box12 modem.
network.wan.password='ppp'
network.wan.device='eth2.6' <- here I add the direct port with vlan 6.
network.wan.delegate='0'
network.wan.ipv6='0'
network.wan.metric='20' <- note this metric is not usual but ive used it for wireguard so connectivity restores if I turn wireguard down.
tl;tr optionally.
also for later would be nice from me to include aswell since alot of information is scarce to set things up for iptv and even normal internet the information site from the isp didn't gave me alot
of information to start with.
the iptv device
uci show network.@device[4]
network.cfg190f15=device
network.cfg190f15.type='bridge'
network.cfg190f15.name='br-iptv'
network.cfg190f15.bridge_empty='1'
network.cfg190f15.igmp_snooping='1'
network.cfg190f15.ipv6='0'
network.cfg190f15.ports='eth2.4'
the iptv interface
uci show network.@interface[9]
network.iptv=interface
network.iptv.proto='dhcp'
network.iptv.vendorid='IPTV_RG'
network.iptv.device='br-iptv'
network.iptv.delegate='0'
igmpproxy (you have to restart the router afterwards)
uci show igmpproxy
igmpproxy.@igmpproxy[0]=igmpproxy
igmpproxy.@igmpproxy[0].quickleave='1'
igmpproxy.@phyint[0]=phyint
igmpproxy.@phyint[0].network='iptv'
igmpproxy.@phyint[0].direction='upstream'
igmpproxy.@phyint[0].altnet='0.0.0.0/0'
igmpproxy.@phyint[1]=phyint
igmpproxy.@phyint[1].network='lan'
igmpproxy.@phyint[1].zone='lan'
igmpproxy.@phyint[1].direction='downstream'
firewall rules and zones
uci show firewall.@zone[8]
firewall.cfg1ddc81=zone
firewall.cfg1ddc81.name='iptv'
firewall.cfg1ddc81.input='ACCEPT'
firewall.cfg1ddc81.output='ACCEPT'
firewall.cfg1ddc81.forward='REJECT'
firewall.cfg1ddc81.masq='1'
firewall.cfg1ddc81.network='iptv'
uci show firewall.@forwarding[10]
firewall.cfg21ad58=forwarding
firewall.cfg21ad58.src='lan'
firewall.cfg21ad58.dest='iptv'
uci show firewall.@forwarding[9]
firewall.cfg1ead58=forwarding
firewall.cfg1ead58.src='iptv'
firewall.cfg1ead58.dest='wan'
uci show firewall.@rule[10]
firewall.cfg1f92bd=rule
firewall.cfg1f92bd.name='allow iptv'
firewall.cfg1f92bd.src='lan'
firewall.cfg1f92bd.dest='iptv'
firewall.cfg1f92bd.target='ACCEPT'
uci show firewall.@rule[11]
firewall.cfg2092bd=rule
firewall.cfg2092bd.name='allow-mcast'
firewall.cfg2092bd.src='iptv'
firewall.cfg2092bd.dest='lan'
firewall.cfg2092bd.dest_ip='224.0.0.1/4'
firewall.cfg2092bd.target='ACCEPT'
so far for iptv you don't seem to need a route set anymore as what they did put on multiple tutorials, it seems it gets pushed by a dhcp option automaticly now.
I hope this can help you finding your issue 
edit:
if you want to use ipv6 just ignore the ipv6 options I have set in the configuration it should work but I have disabled it myself only iptv should be non ipv6.