I use this:

https://www.fachords.com/tools/fretboard-trainer/ (if you choose Easy mode you can select single strings to start with)

I have been using the tool for a few minutes a day (start of practice) for about a year which sounds like a long time but is nothing really in guitar learning terms I don't think and I can honestly say I know the notes very well (even if I'm still only very novice at playing!). When I pick up my guitar I always choose one note e.g. F# and make sure I can find it on every string. The two methods combined have been great.

I think it's the best fretboard trainer I found but I'm sure many will disagree with me!

Hope this helps,

theMolster