I see that the SL1300 is similar to BL1300 with a focus on IoT gateway and using BLE and Zigbee. Very cool but I think that you are hitting difficulty because you are trying to connect two gateways. It could be as easy as switching your hierarchy. BL1300 as Prime and SL1300 Second. BL1300 as ‘router’ and SL1300 as ‘gateway’. Semantics aside that could be best option. Following FW evolution to match required function. These things are only as smart as let them be.
If you start leaving a dent in the wall later… pick up cheaper Mangos and use those as repeaters or extenders. The sip power and nearly industructible.