Activation stuck at step 4

I am now trying to activate The Things Gateway (Model: TTN-001-915-1.0) but having a trouble.
I went through step 1, 2, and 3 but got stuck at step 4 on the activation site (https://activate.thethingsnetwork.org/).
I can see the gateway LED1 On, LED2 Slow blink: “Connecting to the internet” over 10 minutes and no further progress. I tried multiple times but there was no difference.
Can anybody please give me the idea how to fix this issue?
Thank you.