I need a DNS technician

I have a domain websit domain.com, and I have an internal server 2016 with the same name domain.com I placed an A record to www.domain.com in the DNS of the server and it gets there just that the HTTPS comes up on the URL and states :not secure. I have tried the a CNAME, an A record for the internal domain, I've tried a forwarder. Same issue. I can however access the website outside the building, and it works fine.