Just wanted to help others. I spent 2 days dealing with this. I have a hot spot with T-mobile and use that for my Internet connection.
I recently tried to setup Blynk on a nodemcu but no joy. Everything looked good. The DNS resolved correctly. No ports/ip addresses were blocked on my router. I could successfully telnet to Blynk-cloud.com on 80. The arduino IDE console showed nodemcu connecting successfully to WIFI and getting an ip. The nodemsc could ping blynk successfully. My router showed the nodemsc sending traffic to blynk-cloud.com, but zero inbound traffic to the nodemcu. The arduino console consistently repeating “Connecting to blynk” but no connection. Drove me crazy.
Anyway because some other commercial IOT devices previously didn’t connect to the cloud (I had switched to use a local server for them), I suspected that either the IPv6 address that T-mobile gives users -what else could the issue be?
Anyway, I broke down and bought a cheap VPN. All my Internet traffic now goes through that and I now have a IPv4 outgoing address. And everything works.