Device: Not all devices support all kinds of networks. Checking your phone supports 3G, 4G and 5G is a good place to start.
Weather: Adverse weather conditions, such as heavy rain or snow, can sometimes interfere with signal transmission.
Location: Certain indoor locations can cause weak or intermittent signals e.g. in an elevator or underground car park.
Movement: When driving, your phone has to leapfrog between cell towers, which can affect connectivity.
Geography: Signal strength will weaken as you get further from a cell tower. Mountainous regions and even tall buildings can also obstruct signals.