@Dormen welcome to the community.

Can you also provide a picture of your network layout about how you tend to connect all the Deco units?

(Please mark out the model number of the switch if there is any.)

Thank you.

Best regards.

  @Dormen 

Thanks for the update.

I see you mentioned when the 3rd Deco is powered on no matter it is wired or wirelessly connected, it is not possible to see the SSID.

Do you tend to say the SSID actually is detected but no internet service when connected?

But if you mean the SSID physically disappeared, it is quite expected and have you checked on all your mobile devices?

Supposing it is connected but no internet and the whole Mesh turned red, if the main Deco and 2nd Deco are connected like this:

When Deco is on wireless router mode, only the main M4 could be connected to TL-SG1024D.

(Unless your layout is actually  like: ISP modem--main M4---TL-SG1024D, the switch is behind Deco.)

If you wish to connect the 3rd M4 to SG1024, you might need to change the operation mode to the access point, which is also the possible reason why the issue happened.

But since you mentioned you have tried to wirelessly connect the 3rd Deco, then the problem should not happen. so if it is the case now, we might need to discuss it later and collect more details.

Then here comes another situation:

Under access point, you could have more than one Deco unit connected to the switch and the current issue should not exist either.

---Since you mentioned in the first post "I installed 2 of 3 Decos in my network as APs.", I am not sure whether the "APs" here means the working mode or it just simply stands for Deco that is used for wireless extension. I believe it is the latter.

So far, there is still some uncertainty and I hope you could take some time and help me figure out which one is more like your case.

Thank you very much.

Best regards.