topic Issue with 5m Ethernet cables not working (ONT to router) in My Router

Issue with 5m Ethernet cables not working (ONT to router)

AndyZ — Tue, 23 Sep 2025 19:23:08 GMT

Hi all,

I’m having a strange issue with my setup and would appreciate any insight.

Setup: ONT → Router
Working fine until: A few days ago (been running for 6 months)
Cable used: 5m Cat7
Issue isolated to: either ONT or cable

What I’ve tested so far:

Original Plusnet Cat5e (1–2m) → Works fine
New 5m Cat6 → Doesn’t work
New 5m Cat5e → Doesn’t work
Existing 5m Cat7 → Stopped working

Other checks:

Tried three different routers (Hub 2 + two different Deco units) → Same results replicated every time
- Short 1m Cat5e always works
- Any 5m cable fails

So essentially:

3 × 5m cables tested (Cat7, Cat6, Cat5e) → None work anymore
1 × 1m Cat5e → Works every time

Everything I know about Ethernet suggests this shouldn’t be possible, so I’m out of ideas.

Has anyone come across something similar or have any suggestions on what else I could try please?

Re: Issue with 5m Ethernet cables not working (ONT to router)

spile — Wed, 24 Sep 2025 06:52:29 GMT

Power on/off ONT and Router after changing cable?

Re: Issue with 5m Ethernet cables not working (ONT to router)

HPsauce — Wed, 24 Sep 2025 07:51:07 GMT

This suggests a faulty ONT.

Re: Issue with 5m Ethernet cables not working (ONT to router)

quelquod — Wed, 24 Sep 2025 11:23:45 GMT

Or a faulty/marginal socket at one or other end?

Re: Issue with 5m Ethernet cables not working (ONT to router)

krusty — Wed, 24 Sep 2025 12:19:56 GMT

@AndyZ try connecting the failed 5m cables from the router to a pc/mac/pi

Re: Issue with 5m Ethernet cables not working (ONT to router)

krusty — Wed, 24 Sep 2025 12:20:22 GMT

@HPsauce wrote:

This suggests a faulty ONT.

or faulty cables

Re: Issue with 5m Ethernet cables not working (ONT to router)

Baldrick1 — Wed, 24 Sep 2025 12:29:23 GMT

I would suspect the contact pins in the ONT being suspect and not contacting properly. A good test would be to use an Ethernet coupler to add one of the non working cables on the end of the working one.

Re: Issue with 5m Ethernet cables not working (ONT to router)

Jones — Thu, 25 Sep 2025 17:28:05 GMT

@AndyZ ,

I had a similar experience.

I wanted to move the router to a more convenient place than the OR engineer had put it, so after he left I replaced the cable that he used with a longer one. The new cable worked, but the broadband speed dropped to two or three Mbps. A few hours later I put his cable back. There was some improvement, but not much.

I noticed that the plug on the end of the cable that I had used had white stuff on the terminals. I disconnected the ONT and took it off the wall. The strip terminals inside the ONT ethernet socket were covered in the same white stuff. It took me a while to work it out, but I believe that the terminals had become contaminated by the filler that the OR engineer used to fill the hole that he drilled in the wall. The hole is just behnd the ONT ethernet socket. The filler hadn't set when I put the longer cable in. and I must have picked some up when I replaced the cable.

I naively bought a new ONT on eBay, but of course, it didn't work. I didn't want the OR engineer back, so I decided to try a cleaning job on the one that he fitted. I used a cotton bud dipped in IPA (isopropyl alcohol, not the beer) and carefully wiped the terminals. I wasn't optimistic, but it worked. I now get the full 900Mbps.

Re: Issue with 5m Ethernet cables not working (ONT to router)

AndyZ — Thu, 25 Sep 2025 18:26:01 GMT

Thank you everyone for your ideas and comments. I have added an ethernet coupler so the original shorter Cat5e cable is attached to the ONT, which is then connected to a 5m cable attached to the Hub2.

I guess the correct thing to do now would be to contact Plusnet with this to look to get the ONT replaced? But for now I'm happy with this set up - the coupler is hidden behind a unit.

Thanks again all and @Baldrick1.

Re: Issue with 5m Ethernet cables not working (ONT to router)

krusty — Thu, 25 Sep 2025 21:30:39 GMT

@Jones wrote:

I naively bought a new ONT on eBay, but of course, it didn't work. I didn't want the OR engineer back, so I decided to try a cleaning job on the one that he fitted. I used a cotton bud dipped in IPA (isopropyl alcohol, not the beer) and carefully wiped the terminals. I wasn't optimistic, but it worked. I now get the full 900Mbps.

just a side note to anyone else thinking "oh i will just buy an ont from ebay"........ it won't work as OR takes the serial number of the ont and phones back to OR and they added into the local pon for your road and or adjacent road and then the ont is added to the pon.