Bluetooth Low Energy MTU size for characteristics


I am creating a BLE application based on the NordicThingy sample.
The sample only uses characteristics that are short.
I have a BLE GATT server that has a characteristic which is ~150bytes.
I can connect to it and everything works but the value I receive is truncated to 20 bytes.

After some research it seems that 20 bytes is the default MTU size.
I can receive the full longer value using the android "nRF connect" app though.
So the problem seems to be my app.

Does anyone know how to solve this?