Is running two ethernet cables through single-port powerline adapter possible?

Question

I have a house with two floors and the following setup which works great:

Even though I have internet via wireless from the router on the 2nd floor, now I'd like to also have LAN connection on the first floor at the same location as my IPTV STB but installing cable from the 2nd to the 1st floor is unfortunately out of the question.

Theoretically there seem to be at least two ways to do it:

1. Combine IPTV and LAN cables into the single cable either by using RJ45 Y-splitter or by manually making Y cable where 2 pairs would be used for LAN and the other 2 pairs for IPTV (this would limit LAN speed to 100 Mbit but my internet speed is way below 100 Mbit anyway). Is it possible to send two separate LAN signals through a single-port powerline adapter like this?
2. Use switches with VLAN tagging. I'd like to invest minimum amount of money to make it work and since I have a couple of old routers laying around I thought I could use them as switches but I dont know if it's even possible to make VLAN trunking (combining two VLANs into the single LAN port) with low-end routers like this?

Answer 1

1. Get a power line adapter that supports two or more simultaneous ethernet connections. Something similar to this https://www.tp-link.com/uk/products/details/cat-18_TL-PA8030P-KIT.html

2. check the compatibility lists for dd-wrt, openwrt, and tomato. One of your old routers may be in the compatibility list. Depending on the model and chipset of the router it may be vlan compatible running non-oem firmware.

Note: if you don't know what a flash process is, get someone to help you get third party firmware on one of your old routers. If you slip up doing it, you may find you have a nice router sized paper weight in your home.

Answer 2

I (somehow) made it work using an old router which was collecting dust laying around even though theoretically it shouldn't work. In case someone else might stumble upon the same question I'm going to post my working configuration here even though it's currently hardware specific one.

The solution uses VLAN tagging capabilities of both TP-Link TD-W8901G and ZTE ZXHN H108L routers but I'm still puzzled why it works even though none of these routers support VLAN trunking. Both of these routers have almost the same web interface which led me to believe that the underlaying firmware is also almost the same.

Wiring of the working network setup is the following:

Second floor
------------

TP-Link TD-W8901G (router, acting as an ADSL modem and now also as a switch) 
  # RJ11 - connected to the ADSL splitter
  # LAN1 - connected to the ASUS RT-AC58U WAN port
  # LAN2 - n/c (IPTV used to be connected here via powerline)
  # LAN3 - connected to the ASUS RT-AC58U LAN4 port
  # LAN4 - connected to the TP-Link TL-PA4010 powerline adapter (on the same floor)

ASUS RT-AC58U (router)
  # WAN  - connected to the TP-Link TD-W8901G LAN1 port
  # LAN1 - connected to the ASUS RT-AC1200G+ (which acts as an access point)
  # LAN2 - n/c
  # LAN3 - connected to the Raspberry Pi (for smart home use)
  # LAN4 - connected to the TP-Link TD-W8901G LAN3 port

TP-Link TL-PA4010 (powerline adapter)
  # LAN - connected to the TP-Link TD-W8901G LAN4 port


First floor
-----------

TP-Link TL-PA4010 (powerline adapter)
  # LAN - connected to the ZTE ZXHN H108L LAN4 port

ZTE ZXHN H108L (router which acts as a switch only)
  # LAN1 - connected to the IPTV STB
  # LAN2 - connected to the Android smart box
  # LAN3 - connected to the Samsung Smart TV
  # LAN4 - connected to the TP-Link TL-PA4010

VLAN configuration on the TP-Link TD-W8901G router/modem/switch:

VLAN rules / VLAN port assignment

VLAN configuration on the ZTE ZXHN H108L router/switch:

VLAN rules / VLAN port assignment

Basically what I did was create VLAN 7 which would carry the IPTV stream and VLAN 8 which would carry the local network. Since TP-Link TD-W8901G router already has support for VLAN tagging I decided to "re-use" it also as a switch so that's why I brought local network back from the ASUS RT-AC58U to its LAN3 port and since IPTV was already in this TP-Link I decided to output both VLANs on its LAN4 port.

Then on the ZTE ZXHN H108L I split incoming VLANs on LAN4 port back into two separate networks and assign each VLAN its port on the router/switch (LAN1 for IPTV, the other 2 LAN ports for local network).

After everything was working great (IPTV on the STB and local network/internet on both Android smart box and Samsung Smart TV) I verified that I get IPTV signal only on LAN1 of the ZTE ZXHN H108L router/switch as expected. So the setup works but I'm still wondering why?