Recently and since updating to Mojave I have had a frequently occurring problem of the computer not recognising the keyboard or mouse when waking from sleep. Both are Apple devices but when I try a non-apple keyboard it does not recognise that either. Sometimes if I connect the keyboard with its charging cable it will work but not always. Even trying to pair the mouse or keyboard again does not work. However, restarting the computer solves the problem.
I presume you have un-paired/re-paired the devices already.
There are some Bluetooth "reset" options you could try: You need the Bluetooth icon on the menu bar (turn it on in System Prefs>Bluetooth if it's not already there.
Hold down Shift and Option then click the Bluetooth icon on the menu bar to get a longer menu than usual containing a 'Debug' item. This gives several things worth trying; they're fairly obvious but there's more info here.
Disclaimer: I haven't tried any of this myself, the wonderful Sierra (and its successors) with its competent wireless stack finally made Bluetooth work properly for me.
Lionel Ogden 