Hi,
Be sure to backup your original configuration just in case.
For you 1st issue, I see you use static IPv4 address. I would try to use DHCP to get your WAN address:
iface eth3.836 inet dhcp
And remove other parameters on the block such as address, netmask, broadcast and gateway. Keep hwaddress.
From my experience (also on with a GNU/Linux home made router), I had also this kind of disconnection, I switch to dhcp6 and problem disapear.
For your second issue, if your IPv4 internet is working via DHCP, it seems you are on "old" Free infrastructure. So to have IPv6 you need to setup a 6rd tunnel. I may be wrong but being on P2P infrastructure does not change anything for IPv6.
Please note that on "recent" Free infrastructure deployment, IPv6 is native and IPv4 goes by a IPv4-over-IPv6 tunnel. You may be migrated one day though so remember that

. FYI on the forum you will find my notes to setup a Debian GNU/Linux box for this kind of infrastructure.
Regards,