Well, I don’t know how people used to handle this problem before the internet was a thing, but smartphones have made it way easier to get help on the road. The last time my truck broke down on the road I just pulled out my phone and looked up “semi truck repair near me”. Search engines will check your GPS location when you run searches like that, and then show you results relevant to your location.
In my case, that search helped me find this website, which was useful. If you are lucky you’ll be able to find a mechanic who can drive to your location to fix your truck. Otherwise, it’ll have to be towed to the nearest shop, which can be both costly and time-consuming.
Speaking of which, It’s also a good idea to look up phone numbers from tow companies in towns near your breakdown location. For obvious reasons, tow company drivers will be familiar with local auto-shops, and they are likely to know who the nearest diesel engine mechanics are.