I did make photos many months ago but never got around to editing and posting them because I thought I should write a lengthy explanation too.
Here you have them in an imgur album with explaining text in the description:
About the cables:
TP1 is RX of the router. So you should connect TX of your TTL dongle to it.
TP2 is TX of the router and thus needs to be connected to RX of your TTL dongle.
GND of course needs to be connected to GND of the dongle so both your dongle (and thus PC) and the router share a common ground.
And you never ever connect any Vcc of the dongle and the router with each other. On one hand the router has its own power source and on the other you can definitely damage the SoC if you supply more than 3.3V to it. Further more you don't know what other parts of the router you would be powering via the dongle and that might lead to insufficient power and possibly bricking the device.
Also don't connect a 5V TTL dongle to it. I don't know if the pins attached to TP1 and TP2 are 5V tolerant and neither do you. Definitely test the dongle first with a multimeter.
And after that do you know what you're doing or do you need explanations for that too? Because so far I've only written that to someone who has a bunch of Deco P9 and even a Deco M4R V3, but it's all in German.