@SeoulGal 

Most of the issues you described are because your Deco M9 Plus have very old firmware. It is so old, it can't be updated through Deco app. It has to be updated manually.

Follow Method 3 from this TP-Link FAQ document: How to Update the Firmware of Deco

If firmware update is successful, you only need to do it once. For the future updates, you can use Deco app. After the update your M9 Plus should be on firmware 1.6, and that firmware will allow you to configure how Deco M9 (and M5) connect to each other.

If you'd like to, I can give you recommendations on tuning your Deco mesh, but they will be applicable only with latest M9 Plus firmware.

I can also help you with M5s, if you are interested. My Deco mesh is M9 Plus and M5 units, they do work together but it is important to place them correctly. Again, this I can only do after M9 Plus firmware update.

  @Alexandre. Thank you for that information. We tried it all but couldn't get past step 1 of downloading the required update manually. We got loads of red warnings about using the correct version and site and were taken to several different pages and ended up not knowing what on earth we were doing. It's far too complicated so we've given up now. It shouldn't be this difficult. Time to find a new system but thank you so much for your help. :)

  @SeoulGal 

Firmware that runs on your Deco M9 Plus is more than 3 years old. Since then, TP-Link made a lot of improvements in firmware, based on feedback from people like you. By updating to latest firmware you could have fixed all or most of issues you experience. M9 Plus is quite a good Deco model, I decided to keep mine for longer than I planned to. It runs quite nicely, when on latest firmware and properly tuned.

During the time I am active on this forum I helped about 10 people with updating their M9 Plus from firmware 1.2. About 8 out of 10 were successful. While there were cases where people were not able to complete the update, success rate is high. Those who did often came back to tell how much their M9 Plus WiFi mesh is better with new firmware.

People are saying they have better success with Windows 10 than with Windows 11 when running TP-Link update tool. If you have Windows 10 or Windows 11 desktop/laptop, and want to give it another try, tell me which version of Windows it is (10 or 11) and I'll walk you through the process step by step.

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In regard to your TV specifically. You said you have Satellite M9 Plus next to it. If you can put that Satellite M9 Plus on TV stand right under TV (or it is already there), give me your TV brand and model name. I'll check its specs to find if another option is available to connect TV to M9 Plus. If it is, that might be enough to improve streaming on TV while you are either updating M9 Plus firmware or looking for a different WiFi mesh.

  @SeoulGal That's really kind of you. We've decided to go for a new Deco X75 AXE5400 system. We figured if we start from scratch, we'll have more luck. The problem we have is that we have an old house with really thick walls and wooden beams throughout. We ended up with 6 Deco units spread throughout hoping it would overcome this. The new system is supposedly good for a 7200 sq ft house (we're 5000) so we are hoping it will work with far fewer boxes. We also have 2 Sky boxes and they are a problem as they keep dropping out as the internet shifts around from Deco unit to Deco unit. With the new system we should be able to name each one and tell them which one to connect to. Fingers crossed. If we have any issues I'll come back as you really do seem to know what you're doing. Thank you again. :)