When you say "and I had sex with guy #1 days after," do you mean days after your period, or days after the sex with guy #2? If you mean days after guy #2, the only way it would be possible for guy #1 to be the father is if you have really irregular menstrual months, like sometimes the month is 27 days long and sometimes 40 days long and like that. (A period tracker app is only as good as the woman is regular, unless it has years and years of her data and can predict irregular cycles. I heard of one that good, though.) Anyway, if your cycles come on a regular basis and your tracker has been accurate for a while, I'd trust it, and if you had sex only with guy #2 during your fertile window, there shouldn't be a question. Are you concerned because of the sex before your period? That won't impact anything.
For a standard 28 day cycle ovulation is CD14, in this case Jan 8th. Your period tracker says your fertile window was Jan 5-14, which tells us you have an erratic cycle sometimes as long as 34 days. Even so your unprotected sex with Guy2 went on until Jan 18th, well past the longest ovulation on record. It is highly likely that Guy2 is the father. Sex before your December period cannot be responsible, and after Jan 18 is highly unlikely.