I have actually found the best thing to do is set the devices with static IPs and then remove this Smartapp.
Only issue being you can’t actually remove it so I just deleted everything out of the code and republished it. Since doing that none of my 4 sonoffs have dropped and it has been about 2 weeks now.
It appears that the smartapp gets confused every now and again and updates the DTH with an IP of 0.0.0.0 this is what causes the devices to drop off. I first tried just making them all static (as someone else here suggested) but still had the issue occasionally. Once I disabled the smartapp, everything just worked.
only issue is, if I want to add more devices then I will have to re-enable the smartapp, add the devices and disable it again. Just make sure all the IPs are correct before disabling the smartapp, if any are 0.0.0.0 then just wait until they are updated correctly then do it.