Fibre-Links not working after new fibre cable installed. Help!

Hi all,

I posted this earlier but I didn't have much time to really put a proper post together, and I was on my mobile.

We have a link between 2 sites which went down, previously it ran a cat5 cable 146 meters, point to point (don't ask, I didn't do it).

Last Monday, something happened late at night and most of our switches went down, we had to bring the switches back up and then figure out why the links were 'flapping' when plugged in, turned out it was STP, turn that off, we're back in business.

Now the cable which ran to the other building, the port would flap up and down constantly, even when we turned STP off. The local cabling company came in and ran some tests and determined the cable was faulty as their kit was showing extremely strange readings, wrong pairs, strange disconnects....

We've had a fibre link installed by said local cabling company, they have installed a SC fibre patch panel at each side. Obviously our switches are LC fibre, so we've purchased some SC to LC fibre cables, plugged everything in, and nothing.

I've pulled the switch over from the other building and plugged them both directly into each other, bypassing the fibre link in-between, both SFP ports come up and everything is acting normal.

But when we run this through the newly patched cable nothing works at all.

We've swapped the fibre cables over on both sides of the patch panel, tried different SFP ports, replaced the SFP modules, still no link light on either switch.

We are using NetGear AGM731F 850nm 1.25GBd 1000Base-SX/LC SFP modules.

I have confirmed from the cable that the cabling company have laid a multimode cable, and that we're using multimode SC to LC cables from the patch panel to the switches.

I have followed the fibre light from the main switch over to the second building, and in the SC fibre panel, I can see that the light is there, I have then run this cable into the switch, ensuring that the light cables do not clash, still no link light on either side.

On the switches, we don't even get any logs to say anything is there, we get a log to say that the SFP module has been plugged in, but then when we plug the fibre in, it's as if it doesn't exist.

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We have one of these on either side:- https://www.batna24.com/en/p/hpe-office-connect-1920s-48g-4sfp-switch-rmmjj

Does anyone have any idea? Any help would be greatly appreciated.