Ok a couple of questions:
Are your computer and smartphone connected to the same Wi-Fi/LAN or are you using a mobile Internet Plan on your Phone?
If both devices are connected to the same network it's probably a problem with your PC. If they're not then it's likely a Problem with either your Router or ISP.
In any case you should be able to work around this Problem by setting your PC's DNS server to
- 220.127.116.11 (Cloudflare)
- 18.104.22.168 (Google)
- or any different DNS provider of your choice
Cloudflare has a good guide on how to do this on your PC:
Of course this would work with any other provider as well, just use a different IP.
Note that in case you're currently accessing your router's settings via a webinterface (something like
http://my.router), changing the DNS entries on your computer will make this site unavailable from your computer until you change back your settings.
Let me know if that worked for you