Last night I had a similar problem that my VOIP light goes red, DSL light goes off, then blinks & reconnected like every 5 to 10 minutes even after I reboot the modem & router several times.

I did NOT manage to check my router log but guess it's possible the SYN flooding attack hits the Huawei modem at the very first place while the SYN flooding & Xmas port scan attack prevention is set at the router (DIR-615).

Guess TM should really make some configuration to the Huawei modems to prevent such things from happening.

Anyway, currently this morning, so far it is ok.

This could very well be the reason:

VoIP flooding attack

You have to choose a new number since the phone line by UniFi is VoIP while your existing phone line is PSTN.

E.g. those under TTDI exchange, the PSTN phone number is 03-772x xxxx but with UniFi, you will get this number 03-773x xxxx

It looks like tonight just like yesterday night, I have the same problem again:

VOIP light turns red, and DSL light goes off, then blinks & stable and reconnected.

This happens like every 10 to 15 minutes.

I am still wondering if this is a cable problem or a DoS attack.

After all, if its a cable problem, why is it the problem does not occur during day time?

yes. the VoIP number is tied to UniFi. So you terminate it, there goes your number.