As said, GPS
is the way it works. Apple
should and Android
definitely does work for GPS in Flight Mode (important as that's a big battery saver).
I know nothing about Apple apps, but
GPS Tracker by
FollowMee is free and should at least tell if you if your phone is working as a GPS receiver/tracker, so perhaps give that a shot.
I didn't need to use a Sim to get my donated Android (and subsequently drowned on Ben Lomond) phone to use and track via GPS, but I did need a Sim to activate the phone after a factory reset. RiffRaff, sorry,
GiffGaff are cheap and non-contract, it cost a tenner and it meant I had a tenner's worth of phone-calls/data/texts on it (not time-limited either).
Speaking of which, if it's not fully waterproof get a waterproof case or ziplock bag for it.
As already said, if you're going anywhere fruity / unknown you should navigate with a map & compass.
Even if you don't have credit Apple have an
Emergency SOS feature, something about pressing the power button five times in a row. You might need to adjust settings,
read-up for yourself.
If your phone is not up to it, you can get a weak 2nd hand "rugged" (waterproof & shock resistant) Android phone for under £50 on EBay (e.g. Samsung
Xcover 3); though before I got a donation of a dedicated GPS unit I was looking at importing a new
Ulefone Armor 6e from HK for about £150.