UPDATE: Changing MTU doesn't solve the Google issue at all. I tried different MTU value, from 100 to 1500. I still couldn't visit Google and Google-owned sites.
I got frustrated and reset the router to factory default. It worked for one night. But now, I can't visit google.com and gmail.com. I can't even ping to these sites:
ping gmail.com
Ping request could not find host gmail.com. Please check the name and try again.
ping google.com
Ping request could not find host google.com. Please check the name and try again.
ping bing.com
Pinging bing.com [204.79.197.200] with 32 bytes of data:
Reply from 204.79.197.200: bytes=32 time=8ms TTL=117
Reply from 204.79.197.200: bytes=32 time=14ms TTL=117
Reply from 204.79.197.200: bytes=32 time=9ms TTL=117
Reply from 204.79.197.200: bytes=32 time=15ms TTL=117
Ping statistics for 204.79.197.200:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 8ms, Maximum = 15ms, Average = 11ms As the above code shows, I can ping to other sites such as bing.com and not these two sites.
Interestingly, only google.com and gmail.com are affected, YouTube and Google Photos are not. As for YouTube, the thumbnails are working correctly, albeit a little slow to show up.
Although right now I can access Gmail through IMAP, it still is a problem as I'm using Google Alerts and Google Alerts need to go through google.com to redirect to the actual site.
I hope this issue will be fixed soon. For the time being, I can use my mobile connection to visit the mentioned sites.