Connecting a DHCP client to DSL bridged modem using VLAN

So Let's start with this diagram. (Note that none of equipment can be moved.)

I have 2 rooms. In Room#2, there is a router (RT#2), connected by a 10m CAT6 cable as PPPoE interface to DSL modem which is in bridge mode itself and located in Room#1.
I want to connect RT#1 which is in Room#1 to RT#2 (RT#1 is DHCP client here and RT#2 is DHCP server.)
Clearly I need to wire another CAT cable from Room#1 to Room#2. However, I want to ask that can I connect RT#1 to DSL modem (using red wire) instead of connecting RT#1 to RT#2?

I heard something about VLAN but I am not sure how this can help in this scenario.
My DSL modem supports VLAN feature, here is the VLAN I defined in DSL modem (Ether1 is connected to RT#1 and Ether2 is connected to RT#2):

I don't know how to go further in RT#1 in order to make that as a DHCP client for RT#2

VLAN setting in DSL modem:

If your router #1 has more than one LAN port, is powered by OpenWrt (or another sane firmware supporting VLAN config), then you can get away with only one cable between the two routers.

You do not need to enable VLAN tagging on the DSL modem, the VLAN configuration is entirely done between the two routers. You need a VLAN Trunk, i.e. this single cable carries two VLANs (the one to the DSL modem and the LAN). All VLAN handling is performed by the two routers.

In short:

LAN 1 -> untagged VLAN 10
LAN2 > tagged VLAN 10 and VLAN 20
All other LAN ports -> untagged VLAN 20

LAN1 -> tagged VLAN 10 and VLAN 20
All other LAN ports -> untagged VLAN 20

Then you can use the interface associated with VLAN 10 on RT#2 for your PPPoE connection and the interface associated with VLAN 20 on RT #1 as your DHCP client.

Can you please show me a diagram?

No, I'm bad at sketching. Think it through and do a web search for VLAN trunk.