Since this is about security, downgrading is not advisable and you better move the problem to the .net application asking them to improve their security.
Yes: you can host your application on premise. Alternatively, you may be able to migrate to Mendix Cloud v3 (since there is no mention that these protocols have been disabled in that cloud), and you could submit a support ticket for that. Note, however, that both of these protocols are considered unsafe, and so they are no longer supported on the Mendix (v4) cloud, see here:
"January 28th, 2019
TLS v1.0 & v1.1 Disabled for Mendix Cloud v4