Visualisation of the frequency coverage. The spectra originate from the observations of E, which are divided by two (upper panel) or three (lower panel) and shifted on the temperature scale for illustrative purposes. Upper panel: The four setups used in the APEX observations. Lower panel: Setups (in colour) of the new observations conducted with the IRAM 30m telescope. Grey spectra show the additional frequency bands covered by the on-off observations of Tiwari et al. (2020), which are used to supplement our data at the position of clump E.