Connection between outside cable and inside

Hello all!
The telegraph pole wire connects into a black sausage shaped box on the front of my house. From there there is internal CW1308 which runs for about 10 metres straight to the master socket. I get a good connection for my distance from the cabinet. My neighbours across the way have external (black) CW1308 from outside their house which connects to CW1308 internal (white) cable at some point and from there straight to the master socket. Their connection drops quite regularly, and despite the checker saying their connection should be a little faster than mine, it is 15mbps lower. Their line is shorter by about 50 metres as the cable runs up their side of the road and down mine. Is there a way I can speed it up for them at all? I know I shouldn't mess with anything before the master socket but I have changed out my Mk3 socket for a Mk4 without issue so am happy to help them if I can. What sort of connectors are used to join external CW1308 to internal CW1308? Thanks everyone. 👍

Jelly crimp or junction boxes, but fiddling with them is a no, only do so if you are happy to pay any future bill to correct any mistakes you make

A junction box or jelly crimp should not introduce a 15 Mbps drop unless a connection is bad. Have all things like extensions, DLM, poor modem being ruled out.

Thanks for the reply Andrew. I know I shouldn't mess with it but it's nowhere near what it should be. There is only one socket in their property. The line stats have the noise margin at 3.5db on the download, and 6.0db on the upload. Their upload is 10mbps. Mine is 15mbps. As I say their line is shorter and they're connected to the same cabinet. Tried changing the modem out (old Sky SR102) for the Q hub which showed no improvement. I'm wondering if the joint between the external and internal isn't in a jelly crimp but some other less effective setup. Other than that I'm not sure what else to look at.

The Q hub did have a bit of poor reputation for VDSL2, and the SR102 while stable could give slow syncs.

Time to see if a different modem syncs at better speeds.