The TSCH (Time Slotted Channel Hopping) protocol from the IEEE 802.15.4-2015 standard is known to be suitable for highly reliable applications in low-power networks of severely constrained wireless embedded devices. Most of the research on TSCH has focused on the 2.4GHz frequency band. The present work extends the TSCH protocol to low data rate applications using the sub-GHz frequency bands for an increased link budget. We introduce multiple improvements on top of the standard TSCH, namely, a special schedule for the network's root nodes and their direct neighbors, as well as the option to have multiple root nodes in a single TSCH network. Experimental results in a testbed and a real-world deployment show that after applying the improvements, the network meets application requirements and provides reliable and energy-efficient operation.