Using a cell phone for GPS is commonly done and is reliable enough for many purposes. Generally you will want to carry a battery bank to recharge your phone. Phones are not as robustly constructed as dedicated handheld GPS devices so if you will want to protect your phone with a case, waterproofing etc.
Generally I used my phone, printed maps and a compass. I use the phone as a secondary device but most of my trips are on well maintained trails where "navigation" as such is not so critical and I prefer the broad overview a printed map can provide.
The phone GPS works with no cell service but you have to make sure you have cached the maps for the area you are in with whatever mapping app you use. There are various mapping apps available which can present all kinds of maps. Even Google maps can be cached for offline use although they are not really recommended for backcountry navigation. Generally you will want to use a paid app for offline mapping. There are several good options depending on your purpose.
That said, if you are going into the backcountry out of cell phone service you should consider getting a satellite messenger such as a Garmin InReach (active paid subscription required). These devices also have GPS and at the very least allow you to send your location and summon help if you get into difficulty. Some require your phone to do more than SOS which is why I prefer the Garmin InReach models. They are self contained and can act as a backup to your phone should it cease to work. Even the Mini which does not have mapping, can provide you with a location which you can find on a printed map should you need to.