When you haven't used the "app" for a while, you get signed out. When you sign back in, you got sent to the home page of FusionPBX instead of the web texting web app. This leads you to either reload the "app" (force closing it and relaunching it) or manually tap the hamburger menu in the top right corner and go to Applications > Web Texting.
This is on iOS v18.4, but I believe the issue is with FusionPBX not Safari or web texting, this issue might have been fixed in newer versions of FusionPBX. unsure at this point, willing to help test and gather more info if requested