The quick answer to this is:- If your home or office 5G router is installed in a location with poor 5G signal and you have much better 5G signal outdoors then it is likely that an outdoor 5G antenna will help you with your 5G connectivity.
5G radio signals use higher frequencies which don't travel too well through walls or windows so by placing an antenna outdoors you can receive the stronger 5G signal and deliver this along the antenna cables to your router indoors. The downside to installing an outdoor 5G antenna is that you need to try and keep your antenna cables as short as possible - ideally around 5m (but can increase to 10m if you have VERY good 5G service outdoors) and most 5G routers use 4x4 MiMo antenna technology so you will have to run 4 cables from your antenna to your router.
In most cases, an Omni-Directional 5G antenna will do the job, and the gain is usually around the same gain as the antennas supplied with your router, maybe slightly better, so the benefit you are getting is not in the antenna gain, this compensates the fact that it has to deliver the signal along the antenna cables, but the fact the antenna is installed where you have the better signal.
