Pietje

its already my existing solar panel setup which i cant wire to deye inverter

The Deye has a micro-inverter input option on the AUX/Smartload port, for additional solar  input. It might be simpler just to run the panels on a string into the Deye's PV inputs.

Yes the over & under voltage values are adjustable on both sets of inverters of the Sunsynk & Deye. In fact on the Voltronics Infinisolar machine also. It is for these reasons that NRS approval is tested at testing centres like Veritas & that they appear on inverter lists like that of the City of Cape Town. The NRS makes sure that no matter what type of fault is thrown at the Grid interactive inverters that they will island when they are required to. 
You will never find an Off Grid inverter on any of the NRS lists or City Of Cape town list as NRS approval is not required because the device does not interact with the Grid & is not capable of Grid tieing & synchronising. It is regarded as an appliance & rules, guidelines & registration are different. 

@Steve87 @Scorp007 Thanks Guys,  Great answers.  Dont know if you can adjust undervoltage/overvoltage settings for deye inverters, but will place a 3phase undervoltage/overvoltage regulator which will cut off all phases when undervoltage and/or phase lost. 

What a good question. I dont know answer but i am sitting next to you waiting in anticipation for it😊

Steve you are spot on. The disconnect must be within 2 sec. If it would not disconnect you could export power on the healthy phases and the lost phase will get voltage via the transformer to which these 2 are connected. 
Part of the grid code spells out the above condition. No to cater for this condition the lower and upper voltages has to be set correctly. If a too low voltage is set the inverter might not see it as a lost phase when the failed phase is just at a lower voltage. 
This is my take on the question 

I can only speculate how the Sunsynk/ Deye will behave but can tell you how the infiniSolar machine from Voltronics will react. A loss of a phase will make the infiniSolar machine run in off grid or backup mode. Both these machines are grid interactive & I'm willing to bet that a loss of a phase will not allow the Grid interaction or Grid tieing & the Deye & Sunsynk will probably do the same. In other words the machine will switch to Islanding mode. 
I haven't tried it but based on everything I have learnt about Grid tieing in the last 12 months I would think loss of phase means Grid instability which should warrant the Islanding function.