Either that router is not configured to respond to pings, as MrSaffron suggests, or your IP Address has changed since you set up the monitor. You need a static IP Address on Plusnet.
Two questions. Did it happen ever work with this router, and if it did have you bought a static IP Address or not?
The fact it starts with no latency showing at the bottom suggests it simply isn't responding even when you are doing nothing. You would have to start a download pretty sharply after getting a connection for a graph like that to happen. I think it's impossible.
Kindness isn't going to cure the world of all its awfulness but it's a good place to begin. Daisy Ridley.
My broadband basic info/help site - www.robertos.me.uk.
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Connection - AAISP Home::1 80/20. Sync 63790/13596Kbps @ 600m. BQMs - IPv4