There is an option in ChirpStack for US915 and AU915 to select subband and to define single channel in that subband. Does ChirpStack also support single channel selection inside the single subband of EU, AS ?
Within the ChirpStack Network Server configuration you can define which channels must be enabled, so you could set this to a single channel. While it might work, note that single-channel setups have many limitations as you are not only limited to a single channel, but also a single data-rate.