Where there is no signal for a phone call there can often be enough to send a text to emergency services. Register for the emergency text service.
See
https://www.mountaineering.scot/safety-and-skills/essential-skills/mountain-rescue/calling-for-help
Above is Scotland version, but same rest of UK
Carry an emergency blanket, spare warmer clothing and a beanie. First aid kit which would be normal. If you do end up stranded alone. You should at least survive the night. Even if you shiver a little.
Let a friend or family member know where you’ve gone. In walking we’d complete a route card and leave it with someone responsible. In the digital age you could email a map with your route overlaid and expected ETAs. Let them know if plans change. Have an agreed time by which they should contact mountain rescue etc. Don’t forget to let them know when you are safe and sound again.
Ride within yourself and the terrain. Pushing your limits on your own when you know there will be no phone signal is definition of stupid. If you aren’t pushing your limits it’ll be extremely unlikely that you’ll have an accident that immobilises you. Besides if you have a crash serious enough to immobilise you. It may also have broken your phone. Even if there is a signal.
As above have exit plans for walking out if you can. Where are the nearest farms, villages, valleys to descend to (and get help) around the route if necessary?
I speak as someone who hill walked solo and went into the mountains long before mobile phones were a thing. So my planning was always about lettting people know where I’d gone, avoiding accidents to start with, having means to deal with accidents and survive the night and weather, and having exit strategies from my planned route. Self rescue if necessary. It was never about “Nothing worry about I have a mobile on me.”