This can be a multitude of problems.
- 1. your ISP has issues.
2. your physical DSL line has issues.
3. your modem has issues.
4. NIC in the ZS box has issues.
I experienced all of the above in the last 6 or so years that I run a ZS box so I always have a couple extra NICs and DSL modems on hand to verify.
Your test with another router kind of proves that #3 is not the case, so rule out ## 1, 2 and 4.
What is telling you ‘no internet connection’? What is the ppp0 interface status in ZS when it is happening? Can you ping provider’s IP at that moment?