Re-waterproofing winter boots

From my experience with leaky goretex jackets, I'd not waste any more money - before you know it, you're half way towards a new pair of boots.  It could be the seams & / or the facbric which is shot.     If you've already a bottle of the relavent Nikwax / Graingers products, then it's worth a try.  I just accept wet, but not cold are part of the winter cycling joys.

Unless it's the Gore Tex variety your boots are probably goosed. However, you could try the cheap suggestions above.

If they are the Gore ones then dampen them in warm water and use a soft brush to gently rub in diluted Nikwax Tech wash. Rinse well.

You could use the opportunity to give them a wash out inside too!

Either way, get some Nikwax TX Direct spray and... Spray. You can do this while still damp or one dry, but I find damp works better. Once dry hit them with some heat with a hair dryer for a few minutes.
Test by sprinkling water over the boots. If it doesn't bead up 100%, pat dry and spray a bit more TX Direct on. Allow to soak in for a few minutes then give it the hair dryer treatment again.

This has worked for me with Gaerne GTX boots

I've always had good results with Nikwax TX direct wash in (normally used for clothes, but works well with breathable membrane (gore tex/event/sympatex) and fabric shoes (incl nubuck/man made suede) - used it on trainers multiple times.

They also do "Nubuck & Suedeproof" and various leather proofing spray/brush on/wash ins - used these on walking boots with good results.

Muc-off Fabric Protect aerosol. Superb & very easy to apply. Water just beads off. £10 a tin from Halfords, cheaper from Wiggle. I use it on all my other waterproofs too,  rather than wash-in re-proofers.