Data access statement: "Tables from Appendix C (from C.1 to C.15) are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/vol/page Data products presented in this paper are available for download through the EHT Collaboration Data Webpage (https:// eventhorizontelescope.org/for-astronomers/data), or directly from the CyVerse repository. The repository contains the following data products: – Broadband spectrum table with frequency, flux density, its uncertainty, and instrument index (format: DAT). – Sampled posterior distributions of the SED broadband spectral model (A-PL) described in Sect. 4.2 (format: DAT). – Sampled posterior distributions of the SED broadband spectral model (A-BPL) described in Sect. 4.3 (format: DAT). – Sampled posterior distributions of the SED broadband spectral model (B) described in Sect. 4.3 (format: DAT)."