DSL-2730U router's ipv6 problem in Ubuntu

Today I faced a very strange problem. I got a new Tata DOCOMO connection and I wanted to fetch upstream changes from a GitHub repository, with git fetch --all. But all I got was

fatal: unable to access 'https://github.com/<repo>': Failed to connect to github.com port 443: Network is unreachable

I thought that the internet connection has a problem. But then I was able to ping github.com and access that site in my browser, although curl still failed.

➜  io.js git:(master) ✗ curl https://github.com
curl: (7) Failed to connect to github.com port 443: Network is unreachable

At this point I became curious and tried out the verbose curl,

➜  io.js git:(master) ✗ curl -v https://github.com
* Rebuilt URL to: https://github.com/
* Hostname was NOT found in DNS cache
*   Trying 192.30.252.128...
*   Trying 64:ff9b::c01e:fc81...
* connect to 64:ff9b::c01e:fc81 port 443 failed: Network is unreachable
* Failed to connect to github.com port 443: Network is unreachable
* Closing connection 0
curl: (7) Failed to connect to github.com port 443: Network is unreachable

Now, it figures out both the IPv4 address and the IPv6 address but favors IPv6 over IPv4. And it looks like, either the modem or the ISP don't support IPv6 based communication. I don't know how to confirm what the actual problem is. I tried to upgrade the firmware of my DSL-2730U router, by logging into 192.168.1.1. But it kept failing, saying the the firmware image file is too big.

So, I decided to disable IPv6 in my Ubuntu machine and I followed the instructions given here and it worked perfectly. Basically, I edited /etc/sysctl.conf file to include the following lines

# disable IPv6 on this machine
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

and then executed sudo sysctl --system and sudo sysctl -p.