I have been using a Lumia 540 (dual SIM) phone running WP 8.1, update 2 for about a year with no problems. When I'm away from my WiFi, I enable a mobile data connection, and an H or H+ symbol appears in the status bar (just after the network name) to indicate that a high-speed data-connection (of one or another type) is open.
Recently, I've noticed that the H symbol persists in the status bar even after I have turned off the data connection. (Settings > Mobile+SIM > Data connection OFF.) It is still there even after several reboots with the data connection off.
Any ideas on how to fix this?