I tried looking it up as well becase I am having the same problem. All I could find was many, many articles saying the same 4-5 steps with nearly the same exact wording.
Just saying, my phone is an LG Aristo 2 plus
1 was recently updated/installed software needed to be deleted/ some software needed update
2: change ip address through the hidden menu (the instructions did not help me get to my ip address, let alone change it)
3: factory reset the phone
4: disable the app through the same hidden menu (for me the pop up "uNfOrTyNaTeLy, Lg ImS hAs StOpPeD. " and would kick me out of the IMS menu and back to the starting the hidden menu)
5: download some software thing that would delete the IMS from your phone
I'm sorry that this has no way of helping you. But I understand the pain, this damn pop up is popping up as I type every 10 seconds. Good luck!