FTTP Connection drops every minute or so Fritz!box 7590 3490

Hello
New on here with very limited IT knowledge but I�m hoping some kind soul with expert knowledge may be able to point me in the right direction to address my problem.
Have an FTTP package from Zen which worked flawlessly until Saturday evening Jan 18 when the internet became unusable. Spent most of Sunday talking to Zen support who took me through several tests. The identified problem is that my router FB7590 was dropping the connection every minute or so. Replaced cables between ONT and Router and PC. Rebooted ONT, Router, PC. Reset 7590 to factory settings � couldn�t then get it going (which is not surprising as I couldn�t get it going when I first put it inline � eventually I imported the settings from a previous FB3490 and it then worked). Next I took the 7590 out and replaced with the old 3490. Same story on the line disconnections. Finally I connected the PC directly to the ONT and everything worked again. This is as far as Zen could take me being that the problem appears to be clearly my side of the ONT with either the router or with kit attached to it. Speculation from Zen is that it may be a faulty piece of kit connected to the router that is introducing voltage where voltage is not supposed to be and consequently causing problems with the router. I�m tearing my hair out and at a complete loss about where to go next.
Any ideas would be most welcome.
Thank you.

It's a process of elimination which looks like it needs to concentrate on the links through the router. As a first step, did you just swap routers over or did you change the PSU as well? If you just swapped routers, do you have another PSU you can use as a test?

Thought would be to check the clock is set right, am wondering if an incorrect clock is causing the PPPoE session to expire and be reacquired

Many thanks GP and yes I used different PSU's on each router.

Could it be that the FritzBox is configured to monitor some remote IP address to test the link is up, and if it fails to respond within a certain time, it resets the connection?

That would be a dubious thing to do, but I have seen worse.

(If you have a switch with port mirroring, you could put it between the FritzBox and the ONT and monitor the mirror port with tcpdump or wireshark to see what's going on)

Thanks, I cant see where it actually shows the time in the router settings, however in the Region and Language Time Zone section it is set to automatcally set the time as soon as a connection is established.PC time is also set from the internet.

One interesting point that may or may not be relevant is that I notice in the Overview section - Interfaces, it shows DSL as disabled, yet the LED for DSL on the box itself is lit.

Thanks. Unfortunately I have a simple Netgear Switch GS116E which, as far as I can determine, doesn't come with port mirroring.

In reply to a post by rogeroakley:

>however in the Region and Language Time Zone section it is set to automatcally set the time as soon as a connection is established

I have seen some ISPs block NTP for client connections (as "protection" against some forms of DoS amplification). It might be worth setting the router to local time only just to rule this out.

Re the Netgear: the "Smart Managed Pro" devices (e.g. GS108Tv2, GS110TP, GS724Tv4) have a full managed feature set including SNMP, telnet CLI and port mirroring. But I think yours is "Smart Managed Plus" which is very basic by comparison.

Thanks. I will set the time manually.

Fine on the switches. I have a GS308T arriving this afternoon, then it will be a case of me doing homework to figure out how to do what you're suggesting.

Just struck me that it is all rather pointless to try and troubleshoot this if both routers are in fact fried so I'll take one over to my neighbour and hook it up to his connection (also FTTP with same ISP) and see what happens. That would at least confirm the status of the router.

You can�t set the time manually on the FB, it has a default time server entry in network settings which can be changed. If you only have the FB connected to the ONT on the FB LAN1 port, other FB ports disconnected, can you connect via WiFi ok, does the FB still reset?