Fritzbox 7530 wiring issue

[#1]

Thanks to all who previously posted some answers to general questions I had about the new router. Very much appreciated.
”New” router now arrived. In fact it is an unused/unwanted 7530 sold by a Zen internet customer, still sealed.
Issue I have is with positioning and wiring.
Previous Plusnet router upstairs with 15m cable to BT point downstairs. Not ideal I know, but it worked for us. Cable was RJ11/RJ11 terminated, with 4 wires connected (on PIN numbers; 2,3,4,5).
The DSL cable supplied by Fritzbox Differs in 3 ways. First, length is 4m so I cannot do like for like replacement.
Second; the plugs are RJ11 (BT socket)/RJ45 (router end). So even if I wanted to, I could not use my existing cable which is RJ11/RJ11.
Third issue, the Fritzbox DSL cable has only two wires connected. At the RJ11 plug, it’s pins 3 and 4. At the RJ45 (router) end , it’s pins 4 and 5.
Couple of questions, really. First with positioning. Am I being silly having my router connected with 15m cable to the BT socket? I’ve read about signal degradation but don’t know how much of a problem this is.
Second point. If I can continue with the new router upstairs and a 15m cable, what cable do I buy? I can find several choices for RJ11/RJ45 cables but all of them have FOUR wires connected, not two as in the Fritzbox cable.
I don’t have the expertise to know how the BT socket is wired internally and so have no idea if it’s only pins 3 and 4 which are connected anyway. If it is, then it wouldn’t matter if I use a 4 wire or two wire cable provided it is only pins 3 and 4 which are connected internally at the BT socket end. However, if pins 2 and 5 are also connected at the BT socket end and I use a 4 wire cable, The am I danger of inputting signals/voltage into the router which shouldn’t be there?
Finding a RJ11/RJ45 cable with only the entire two wires connected has so far eluded me.
And in case relevant, my BT master socket is 5C with a mark 4 faceplate.
Sorry for the long (rambling??) question, but any help much appreciated.

[#2]

@grandad 

Installing the hub this distance away from the master socket is not recommended but if it worke OK for you...

You only need the two connections from the master socket. 

Why not leave your long extension in place and tag the Fritzbox cable on the end using an RJ11 to RJ11 coupler like this?  https://www.amazon.co.uk/RJ11-Coupler-ADSL-Broadband-Adapter/dp/B0029HMN3E

[#3]

@Baldrick1 

Thank you (and for your replies to me previous thread, too!)

Of the things I’d thought about, an RJ11/RJ11 connector wasn’t one of them. I didn’t think there was such a thing, but should have realised there would be.

So just to check if I may. my existing modem cable (the router end) goes into the female socket. I then take the supplied Fritzbox DSL cable and insert the RJ11 plug into the other side of the adapter with the RJ45 plug into the router.

Irrespective of how the BT socket has been wired and the fact that my existing cable is 4 wire, the connection to the fritz cable (With its 2 wire config) means that I cannot possibly pass any “unwanted” signals or voltages through to the Fritzbox?

Thanks

[#4]

Hi @grandad 

you could just plug the rj11 plug into the rj45 socket.

Dan

Edit: - should have added the only differences between rj11 and rj45 one can have six wires the other can have 8 wires..

So pin 2 of a rj45 is pin 1 of a rj11.

Hope this helps.

[#5]

@grandad 

Yes you just plug the to male ends into the double ended female connector.

However, getting my brain in gear @Dan_the_Van is perfectly correct. You can just plug the RJ11 plug unto a RJ 45 socket. The indexing will automatically pick up the correct pins.

The unused wires are just not connected in the Fritzbox socket and can be ignored.

[#6]

@Dan_the_Van 

Thank you.

I had no idea you could plug an RJ11 into an RJ45

[#7]

@Baldrick1 

thanks. Ditto my comments above to Dan. I had no idea that an RJ11 could go into an RJ45 safely.

So, is there any logic to Fritzbox supplying a DSL cable with only 2 wires connected? Could they have provided an RJ11/RJ45 with 4 wires connected, with the router socket rejecting any data or voltage on the (RJ45 plug) pins 3 and 6?

[#8]

@grandad 

I suspect that you are over thinking it At the BT socket end of the filter there will only be two wires connected so that is the extent of the lines that are used.

I suspect that the answer to your concern is no more than another country (Germany perhaps as Fritzbox is designed and manufactured by a German company AVM) uses R&45 connectors for Internet data or it maintains compatibility with straight routers with no internal modem so it's compatible with a WAN connection.

[#9 · Accepted solution]

@Baldrick1 

Thanks.

Funnily about, I thought about editing my last post to include the words “or am I over-thinking this?”

Exactly your comment. Thank you. 

And what you say about the fact that inside the BT master socket, only 2 wires are connected anyway gives further reason not to fret about the 2 or 4 wire issue. In fact, it appears to be a complete non-issue.

I am very grateful to you.