Medium fiber optic broaband, Super router replacement ?

Hello i was wondering if I could get some help, I have the medium package fiber package and the service is great but quite recently for about 2-3 weeks my download speed has gone from my expected 30+ to 10-17 download speeds and this is everyday it hasn't returned back to its download speed but the upload speed is untouched.

I was wondering if I could get a replacement for my router as this happened before and the router was at fault and after receiving a replacement the problem was gone I have contacted TalkTalk but nothing seems to be done and it can incredibly frustrating paying that extra to get faster download speeds and for it to be like this for weeks