How can I find out who has glided access on my iPhone

No one can see what’s on your phone unless they know your Apple ID and password AND you don’t have 2 factor authentication enabled for your Apple ID, and even then they can only see what you sync to iCloud, not what is on your phone. It has nothing to do with IP address; you don’t actually have AN IP address; your phone changes IP addresses every time you connect to an different network, move to a different cell tower, or even turn your phone off, then on again. IP addresses are thus useless to access your phone.

You can easily see all devices logged in to your Apple ID; Go to Settings, tap on your name at the top. The next screen will list all of the devices that share your Apple ID, and you can tap on any and remove them. But my guess is that there won’t be any other than your phone and a computer or iPad that are yours.

If other people have access to your iPhone or other Apple device, change the device passcode and never share it with others.

Change your Passcode -> Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Change your Apple ID password -> Change your Apple ID password - Apple Support

• Create a strong password for your Apple ID, It is best if your password consists of eight or more characters and include upper and lowercase letters, and at least one number. You can also add extra characters and punctuation marks to make your password even stronger. 
• Never provide your password, passcode, security questions, verification codes, recovery key or any other account security details to anyone else. Apple will never ask you for this information.

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

(if you haven't already done this) Two-factor authentication for Apple ID -> Two-factor authentication for Apple ID - Apple Support

To keep your Apple ID account secure, see the following Apple articles: 

If you think your Apple ID has been compromised

If you think your Apple ID has been compromised - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

See who has access to your device or accounts

See who has access to your iPhone or iPad – Apple Support (UK)

Delete suspicious content from your devices

Delete suspicious content from your devices – Apple Support (UK)

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to find where you're signed in - Apple Support

• if you see devices that don't belong to you, delete from your account.

Personal Safety User Guide -> Personal Safety User Guide – Apple Support (UK)

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)

Never jailbroken your iPhone.