Another option:
Use a Raspberry Pi Zero W running DSLStats, currently costs £9.60 + a Micro SD memory card.
Unit is so small could possibly fit inside the router case & pick up 5 V. power, have not tried this as yet.
Or, put Rpi Zero in case, £15.60 with case and stick (velcro) to rear of router. (the method I use)
Very low power requirement, I use an old mobile phone charger, the recommended 2 Amp PSU is only approx. a fiver but deemed overkill.
RPi Zero W has built in WiFi, no need for an ethernet direct connection which would involve buying a micro USB to RJ-45 LAN adaptor.
Comm's. to the RPi by WiFi using VNC viewer.
I have used this very cheap option for my daughter's line (MDWS cruth211) and son's line (MDWS smpay-37), has worked very well to date.