Make sure your phone is connected to the 2.4 GHz radio when you are adding the device so that the device will be sent the MAC address of the 2.4 wifi.
Hmmmm, If you have not already done it, completely delete the th-10 from your ewelink account and start over fresh with it.
I think I found a solution for you, I had the same issue using a Samsung S8+ connected to my home wifi. I downloaded an app from the App Store called "WIFI Manager"by Kostya Vasilyev. Open the app an look for your wifi network and connect to your network and select the lower channel number. The 5g works on the higher channels (mine was on 48). Once you connect to lower number (mine was Channel 4) network you can connect your devices.