Scaring people into using your services?

KPN is saying that using public networks and can lead to MitM attacks. https://www.kpn.com/zakelijk/blog/geen-openbare-wifi-gebruiken.htm (Dutch, sorry).
My understanding is that the mechanisms in Https can make even 'insecure' networks secure, unless someone is able to inject certificates into your certificate store MitM attacks are basically not possible, right?
Their point is that you should use KPNs GSM networks (5G) because public networks are unsafe.

attacks certificate certificates cybersecurity gsm https inject insecure mitm networks point public store understanding