Linksys WRT1900ACS config for ISP that requires VLAN

Actually a crossover cable can be used.
Gigabit introduced Auto MDI-X, it automatically detects the cable type (crossover/straight).
So with gigabit, it simply doesn't matter.
But I'm curious what happens if a gigabit interface is connected to lower speed one.
Does the gigabit interface still do Auto MDI-X?
//edit
Most 100 Mbit/s device should support Auto MDI-X too.

In the openwrt wiki there is only one ISP listened that uses fiber and vlan id 34 and that is:
Bell Canada Fiber, is that your ISP?
//edit
There are two: Telfort also uses 34.

The VLAN tagging is usually as follows:

  • Phone: VLAN 34

  • Internet: PPPoE over VLAN 35

  • TV: VLAN 36

If Bell Canada Fiber is your ISP you can try VLAN (id) 35 instead of 34.

//edit2
Also both use pppoe instead of DHCP, so scratch that.

No, my ISP is 'Tweak' in The Netherlands. Their instructions on using non-ISP products on their network are (at least the bit to use internet):

The router should make DHCP requests on VLAN34 via the WAN-port (802.1q)
The default route should go via VLAN34.

And as far as I can see from the captured frame, that is exactly what it looks like (DHCP request with 802.1q and VID 34).

You have packets going out, but none coming back because the physical link seems to be down.

The cable from the router to the ONT would be suspect.

Using a 2 pair cable is fine for 100 Mb but often causes complete failure when both devices are gigabit. There is nothing in the standard to fall back to 100 Mb on a two pair cable, so many devices don't do it.