Alright, I managed to find it with trial and error, here is how anyone can do it for Turk Telekom and similar providers.
login via ssh and edit the /etc/config/network as follows:
uci set network.wan.ifname=wan.35
uci commit network
/etc/init.d/network restart
Open /etc/config/network and check that it has the wan as follows:
config interface 'wan'
option hostname 'GL-AX1800-27a'
option metric '10'
option username 'USERNAME@ttnet'
option password 'PASSW'
option disabled '0'
option ifname 'wan.35'
option peerdns '1'
option proto 'pppoe'
option ipv6 '0'