How to properly set up a DSL(PPPOE) connection with ipv6 in Loki?

Question

I don't know where to ask this, so I'm asking it here.
Situation: dynamic IPv6 addresses from ISP, wired modem, home computer.
Symptoms: many web pages don't load; no software is shown in AppCenter except what is installed; sudo apt-get update works extremely slow (it doesn't finish updating after 1 and a half hours). Behaviour is similar in Ubuntu 16.04 and Linux Mint 18 KDE.
In Freya ipv6 is ignored by default and I don't have any connection problems. In Loki I'm connecting with ipv6 by default and I'm having the problems described above.
Changing Network > Advance Settings > IPv6 Settings > Method: Ignore, leaving IPv4 Settings > Method: Automatic (PPPoE) has no effect. I've tried disabling ipv6 by editing /etc/sysctl.conf; /etc/default/grub; /etc/gai.conf; or blacklisting ipv6 in /etc/modprobe.d/blacklist.conf. It didn't work. I've also tried sudo apt-get -o Acquire::ForceIPv4=true update without success.

Answer 1

I solved it. IPv6 doesn't need to be stopped. I needed to add the line "mtu=1492" in the file /etc/NetworkManager/system-connections/Connection name like this:

[ppp]
lcp-echo-failure=5
lcp-echo-interval=30
mtu=1492

After making the DSL connection at first, of course. Then reboot and updates are working, and software is visible in AppCenter.