That would be akin to driving a FIAT with a Ferrari manual in hand. Be happy your device is still reachable at all, you might have messed it up to the point of it needing recovery.
Backup your configuration, run firstboot, reboot and reconfigure your WAN interface with the correct VLAN, preferrably through the GUI.
It's not reachable anymore, I will reflash openwrt. But then I don't know how to config the AP-1300 to work with PPPoE.
From the GUI would it be just adding the PPPoE and the device eth0.835 like here?
And setting the username and passwd given by my ISP, is that all? I tried it before but did not work, maybe I need to reflash and try it again?
According to the device page, the wan port is connected to eth1.
If you need further help, post the default network configuration after resetting/reflashing the device.
As @pavelgl pointed out, your WAN is a different interface on the AP-1300, compared with the C7. So you need to pick/add that one (eth1.835), which sets the correct VLAN, and then add the PPPoE credentials again.
This has an ipq4018 chip where VLANs never really worked properly until the DSA conversion with version 23. So make sure you're running 23.05.2. Start from defaults and note that there is no switch section, which means that you can't just copy from a swconfig device like the Archer A7.
Change the wan proto to pppoe, add your ISP login and password, and change the wan device (assuming it defaults as eth1) to eth1.835
Ok since the DSA names for the external ports are wan and lan, use wan.835. You can also remove interface wan6, since when a pppoe ISP offers IPv6, it works a different way.