There's an alternative, but it depends a lot on what the other "LoRa compatible device is".
If it has gateway functionalities (ability to listen on multiple channels and data rates and matching software, usually means SX13xx-based) and you can freely program it, then it may be possible to have the LNS directly embedded in the gateway. In that case you just continue to use full LoRaWAN.
Probably much easier if it's something running some Linux flavour (e.g. a Raspberry Pi of some sort) than a more basic device (e.g. ESP32 based), but it's really just software, so as long as it fits it should be possible.
Depending on your scenario, you probably don't need a full-fledged LNS, so you can probably just build a pretty basic thing with lots of stuff hardcoded if you need to run that on a more limited device.
If the other device is just an end-device (which can listen only on a single channel/data rate at a time, usually based on SX12xx chips), then you have the option to try to turn it into a "nano-gateway", and again, embed an LNS.
The LNS would need to be configured to change the channels sent to the sensor so it only uses the single channel your nano-gateway is listening on.
The join process is going to be clumsy as the sensor will send join requests on one of 3 standard channels at random, while the nano-gateway will only listen on a single channel, but once joined it should work.
Again, it would need the ability to run an LNS (at least a minimal one) on the nano-gateway, see above.
Otherwise, as the other have pointed out, if you really want to use raw LoRa and no LNS at all , then you will need to modify the existing device. Depending on the device, the MCU, whether the firmware source is publicly available, etc, this may be very easy or very complex. Without any details about that device, it's very difficult to say more.