Unfortunately the Signal recommended by many people may not work for you, it cannot work on multiple mobile devices at the same time, for example on a smartphone and tablet.
I don't understand why no one recommends Jami? It's the only messenger that works without a server at all. And yes, it has the ability to forward messages to itself (to use as a note) and works on any devices at the same time.
It works on up to 5 devices at the same time. I’ve used signal on my phone, with it also open on my computer with zero problems. Syncing is instantaneous.
Open Signal rewiews on GooglePlay, there's every third review about this situation.
I forgot to mention, i'm talking about the Android version. The ios version works on ipad at the same time