Urgent: 2 certificates on one domain under IIS

Job ID: 38240667

Budget: €30 – €250 EUR

I need a Domain SSL certificate expert to help me install and configure and concatenate? my 2 certificates on an IIS server 2022.

The ISSUE:

i run Let's encrypt to secure by https the domain. and here all run smoothly.
BUT:
i need also to run another certificate, which is self-generated, in order to accept connections by API to an external server.
The Problem is that this one is not a valid HTTPS certificate so my domain start to show as "unsure".
Maybe it's possible to concatenate the 2 certificates?
Related categories: IIS Windows Server