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.
I've been struggling with same issues on my Google mesh wifi which uses same ssid for 2.4 and 5, like many today. I've managed to get several devices connected but always hit or miss. The "WIFI Manager"by Kostya Vasilyev app is no longer on Google Play. However, I did find this discussion
which I found very informative and his process did at least allow ewelink on my phone to see the 2.4 ghz network. He also points out that itead could do some work on their software to make this problem go away and he gives some insight on how to do it.
From the pocketables link:
" The issue here is these badly written apps are using the WiFi mac address you’re connected to to connect. This means if you’re connected to the 5GHz channel, you’re sending a MAC that’s not the 2.4GHz MAC. "
"While 2.4GHz is most likely better for distance (5GHz is speed,) the designers of these apps should be able to pull all MACs assigned to that SSID just by a quick sniff as opposed to making tons of hoops for most dual-band households to have to go through."
Hope itead developers see this. 95% of users would never go through this hassle to get a product working and I probably won't either in the future.