3G and 4G connections can be patchy at best but if you’re never picking up signal, you might have a problem.
All of the points mentioned above (see What could prevent my phone connecting to a network?) apply here too so take them into account. Are you just in an area of bad coverage? Is your software up to date? Have you tried resetting the phone?
It’s worth checking whether your settings allow for 3G and 4G too. People change their settings and switch off 3G or 4G by accident more often than you think. Therefore, it’s worth checking where your phone’s network settings are located and that they’re set correctly.
While 4G coverage is widespread now, the device you’re using may not be compatible with 4G; this is usually true of older smartphone models. In addition to this, you may not have agreed the use of 4G coverage with your service provider so it may not be in your contract or have been authorised. It’s worth checking that your device is compatible and that your service provider is letting you receive 4G.