Notwithstanding the possibility that your WiFi+Cellular iPad may have a fault, there are other considerations that may impact your iPad's GPS positioning performance. Comparison with iPhone while useful to demonstrate that GPS jamming (an increasingly common issue) is not the source of difficulties, performance comparisons in less than ideal circumstances will favour the iPhone - where antenna positioning is likely better optimised.
For marine and aviation applications, use of a dedicated GNSS/GPS receiver is often recommended. While WiFi+Cellular models of iPad incorporate a GNSS receiver within the Cellular chipset - providing accurate position and velocity measurement for a moving iPad - we see occasional complaint similar to your own.
A typical GA cockpit environment is not ideal - as the iPad's GNSS antenna will not be best positioned for a clear view of the sky. Aluminium and/or carbon-fibre fuselage will block/screen the antenna from direct line-of-sight view of above horizon satellites. Remember, you need at least four GPS satellites with good geometry for a reliable 3D position fix.
Placing a good quality external GPS on the combing - and connecting to iPad via Bluetooth may be a far better solution. For GA Applications, perhaps consider a GNS3000 receiver:
https://www.gns-electronics.de/consumer-products/
Other well known alternatives include BadElf and Garmin GLO - although there are others from which to choose (readily available from your favoured aviation/pilot-supplies retailer). NB: The GNS3000 can also be used as a position logger - fully independent of your iPad and SkyDemon. This may be useful should you wish to maintain an independent track-log for post-flight analysis or archiving.
Returning to your iPad and the possibility of a fault, assuming that your iPad is outside of warranty, a high performance external GNSS receiver may be cheaper than repair of your iPad. You can establish eligibility and cost of repair for your iPad, should it be required, here:
iPad Repair & Service - Apple Support
For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange an iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. If you have benefit of an active AppleCare plan, the cost of repair/exchange is reduced to a small excess/deductible fee.