It does not sound like your ex is the dad, but not by the most safe of margins.
To be sure I understand you, I'll repeat what you said using no acronyms, writing the names of the months rather than using numbers (where I live, 3/11/2019 is March 11) and asking for clarification in brackets.
Last menstrual period [began on? ended on?] October 13, 2019
[Menstrual months are: regular? irregular? and ___ days from day 1 of a period to day 1 of the next period]
Sex with fiance [from October 13] up to October 25
Sex with ex on November 3
[sex was unprotected with both? men]
Learned about the pregnancy on November 12
Took Clear Blue with weeks indication on November 13
showed "3 weeks +"
Ultrasound November 27 showed a 6mm crown-to-rump length; estimated due date was given as July 19, 2020 based on the first day of the last period [did they say why they didn't base it on the ultrasound data?]
Ultrasound on December 26 showed a 40mm crown-to-rump length and gave an estimated due date of July 17
[When you had the ultrasound on November 27, what gestational age count did they give you, was it 6 weeks 3 days?]