What more can anyone say… you are trying to fail-safe your robot the wrong way! Repeatedly hitting the forum with requests for “step-by-step” instructions, for what is in reality an impractical option for what you need, will not help you any.
Think Mars Rover, not RC vehicle. Assume communication between the three main items APP <-> SERVER <-> MCU will fail, in part or completely, at any moment. Make your solution on the MCU side by “waiting for directions before action” instead of “action until communications fail”. This can still be done with reasonable speed.
I don’t believe there is any single “is app on-line” function that will properly suit your purpose. I can think of a few untested hacks with eventor but I have not tried it… nor do I believe it would do you any good, as the premise of that type of APP->MCU “heartbeat” would be a cludgery way of imitating the recommended, “waiting for directions before action” in the first place.
By the way, the “opensource” of Blynk means that you are welcome to dig into the heart of Blynk and make your own modification, not for someone to suddenly 'fess up a “hidden” command that “we” have allegedly been hiding from you 
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