Maybe you’ve already written it, but how far is the router from where you use the iPad? And are there walls between the router and the iPad?
The 2.4 GHz band is massively slower, but the range is significantly better. The 5 GHz is massively faster, but not as great in range. And the speed drops very sharply in the edge area. And one thing you shouldn’t forget is that even if the router still brings the signal to the end device, the end device is already too weak to bring the signal to the router. This can also lead to problems. Only the customer doesn’t see it because the device shows that it is receiving a signal.
I understand that you did the speed tests with the mySwisscom app on the iPad. What happens if you do the tests at the same place with a mobile phone and the mySwisscom app? Same result?
I am still convinced that the problem is not on the IB3 side. The WLAN in the IB3 is a high-performance chipset from Broadcom with their standard drivers that is widely used worldwide. And Apple also (still) uses Broadcom for WLAN and Bluetooth in its end devices. There are no known hardware problems with the WLAN with the IB3. Unfortunately, I only have an iPad Pro 11″ from 2018 or 19. And everything works perfectly there.