@EricBoy 

Do you run your Deco mesh in Router mode or in Access Point mode? This is where to check it in Deco app:

@EricBoy 

In Access Point mode, your setup should be working. I have similar setup to yours and it works just fine:

Router <----------------> Switch <--> Deco Master 

     |                                     \---- <--> Deco Slave #1 

     \------------------- <--> Deco Slave #2  

All are connected through Ethernet.

The only difference is you have one more switch, but I am sure if I plug extra switch I have to replicate exactly your setup, my Deco mesh will still work as it should.

TP-Link recommended changing which Deco will be Main Deco. It is "nice to have" recommendation, but it will not fix the problem. If you tried it, you figured it won't. So, my suggestion would be to either restore your Deco configuration as it were originally with Main Deco, or redraw your setup how it is now and post it so that we know what exactly you currently have. 

I'll copy your original setup again, for reference and put numbers at switches to distinguish between then:

Router <--> Switch #1 <--> Switch #2 <--> Deco Master (Signal source can use Ethernet)

                        |                          \---- <--> Deco Slave #1 (Signal source can use Ethernet)

                        \------------------- <--> Deco Slave #2  (Signal source cannot use Ethernet)

For the problem you have, the usual suspects are Ethernet wiring from Deco Slave #2 to the Switch #1, incompatible with Deco mesh switch or both of them, and also there is a possibility your router is incompatible with Deco mesh Ethernet backhaul.

Is it possible one or both of your switches are from D-Link? See this FAQ article for helpful hints and especially check its Q&A section: General questions about Ethernet Backhaul feature on your Deco

If this article does not help with your situation, you'll need to run series of tests on your home network to narrow down the culprit. If you need help with test steps and order of tests, please ask in your reply and I'll provide you with detailed instructions.

@EricBoy 

> If the new firmware can let me to select or set the signal source, I think the problem is resolved.

It will not. Deco on its own prefers Ethernet connection and only switches to WiFi backhaul if it can't use Ethernet. Even if you are able to tell Deco use Ethernet, it won't be able to, because there is an underlying problem.

You either have Switch #2 incompatible with Deco mesh Ethernet backhaul, or worse - your ISP router is not compatible.

If you would like, I can provide you with series of tests to figure which one is that. If it is switch, you'll be able to replace it by compatible model. Hopefully it is not your ISP router.

If you want me assist you with list of tests, please also provide ISP router brand name, model number and how many Ethernet ports it has.

@EricBoy 

From the TP-Link FAQ article: "The Deco Ethernet backhaul feature is based on the standard IEEE 1905.1 protocol. If you're not sure which switch to use, TP-Link switches may be a good choice."

Sorry, that's all information I have.