Router replacement

I had fibre installed around 2***. the router they provided is 2013 model. On 31 March I reported that my signal was weak, averaging 20 mb with wired connection, paying for 50mb line. I phoned 3 times, each time my identity is confirmed, then the call drops, however, no one bother to call back, so each time you speak to a new person and have to explain again. I was informed there is nothing wrong with the line, they will get back to me, i must send picture of the router.

Monday 4th April, line is still slow and no feedback received, phone again, they will replace router. Then phone me back and say the router is not one of theirs, they wont replace it. The router is what was supp**** by them. I advise that this is ridiculous as they supp**** the router. Should I have taken a picture with the installer smiling next to the router serial number back in 2016?? Surely they should know what they have installed. They will get back to me. Daily follow up mails to *** now go unanswered.