If you don't have a smartphone you can install apps on - perhaps the company making a smartphone app which happens to integrate with a desktop app as well) isn't for you. Same for "tablet users".
FWIW, you don't need to do _too_ much hoop jumping to get a non-phone device running Signal - I mainly use it on a dedicated iPod Touch, and I've got the iPhone app running on an iPad as well - in old-school 2x ugly-mode, but it works. (You do need to be aware of the risks around whatever phone number you use to bootstrap your way in possibly being reallocated. You might not want to use a 30 day burner SIM or a temporary Twilio number if you might have a targeted enough attack to impersonate you by hoovering up temporary mobile numbers...)