It took me a while to figure out how to add fonts to PixelLab.
The problem is that every phone is different. This is how to do it on Xiaomi but it should be similar for other phones. Except for iPhones. No idea how to do it on them.
Version 1: Extracting files manually
Switch to PixelLab.
And that's it. Now your font should be in PixelLab.
Version 2: Using Phonto
If you have difficulties with that, an easier way would be to download Phonto.
Switch to PixelLab.
Then find the folder named "Phonto". It doesn't work on my phone but it used to on my old Android, maybe it can work for you.
Hope this helps! If there are any questions left, feel free to ask.