Try this as last option. Change in Router DNS Server from current ISP to Google DNS Server. You can also use Cloudflare DNS Server which is much better.
Google DNS Server v4:
1. 8.8.8.8
2. 8.8.4.4
Cloudflare DNS Server v4:
1. 1.1.1.1
2. 1.0.0.1
Cloudflare DNS Server v6:
1. 2606:4700:4700::1111
2. 2606:4700:4700::1001