Playing scheduled audio in the background - ios

Question

I am having a really difficult time with playing audio in the background of my app. The app is a timer that is counting down and plays bells, and everything worked using the timer originally. Since you cannot run a timer over 3 minutes in the background, I need to play the bells another way.
The user has the ability to choose bells and set the time for these bells to play (e.g. play bell immediately, after 5 minutes, repeat another bell every 10 minutes, etc).
So far I have tried using notifications using DispatchQueue.main and this will work fine if the user does not pause the timer. If they re-enter the app though and pause, I cannot seem to cancel this queue or pause it in anyway.
Next I tried using AVAudioEngine, and created a set of nodes. These will play while the app is in the foreground but seem to stop upon backgrounding. Additionally when I pause the engine and resume later, it won't pause the sequence properly. It will squish the bells into playing one after the other or not at all.
If anyone has any ideas of how to solve my issue that would be great. Technically I could try remove everything from the engine and recreate it from the paused time when the user pauses/resumes, but this seems quite costly. It also doesn't solve the problem of the audio stopping in the background. I have the required background mode 'App plays audio or streams audio/video using Airplay', and it is also checked under the background modes in capabilities.
Below is a sample of how I tried to set up the audio engine. The registerAndPlaySound method is called several other times to create the chain of nodes (or is this done incorrectly?). The code is kinda messy at the moment because I have been trying many ways trying to get this to work.
func setupSounds{
if (attached){
engine.detach(player)
}
engine.attach(player)
attached = true
let mixer = engine.mainMixerNode
engine.connect(player, to: mixer, format: mixer.outputFormat(forBus: 0))
var bell = ""
do {
try engine.start()
} catch {
return
}
if (currentSession.bellObject?.startBell != nil){
bell = (currentSession.bellObject?.startBell)!
guard let url = Bundle.main.url(forResource: bell, withExtension: "mp3") else {
return
}
registerAndPlaySound(url: url, delay: warmUpTime)
}
}
func registerAndPlaySound(url: URL, delay: Double) {
do {
let file = try AVAudioFile(forReading: url)
let format = file.processingFormat
let capacity = file.length
let buffer = AVAudioPCMBuffer(pcmFormat: format, frameCapacity: AVAudioFrameCount(capacity))
do {
try file.read(into: buffer)
}catch {
return
}
let sampleRate = buffer.format.sampleRate
let sampleTime = sampleRate*delay
let futureTime = AVAudioTime(sampleTime: AVAudioFramePosition(sampleTime), atRate: sampleRate)
player.scheduleBuffer(buffer, at: futureTime, options: AVAudioPlayerNodeBufferOptions(rawValue: 0), completionHandler: nil)
player.play()
} catch {
return
}
}