Emails on Send-only mail server not sending

I've been trying to setup my own send-only mail server so I could send emails using PHP scripts, but I've been running into some issues. Whenever I send a test email to my gmail account, it doesn't go thorugh and I get the following errors in the mail.log file:

Apr 9 06:17:55 driedsponge postfix/smtp[28482]: connect to alt1.gmail-smtp-in.l.google.com[74.125.129.27]:25: Connection timed out
Apr 9 06:18:25 driedsponge postfix/smtp[28482]: connect to alt2.gmail-smtp-in.l.google.com[142.250.11.27]:25: Connection timed out
Apr 9 06:18:55 driedsponge postfix/smtp[28482]: connect to alt3.gmail-smtp-in.l.google.com[173.194.77.27]:25: Connection timed out
Apr 9 06:19:25 driedsponge postfix/smtp[28482]: connect to alt4.gmail-smtp-in.l.google.com[64.233.177.27]:25: Connection timed out
Apr 9 06:19:25 driedsponge postfix/smtp[28482]: 57B4E26099E: to=<>, relay=none, delay=1985, delays=1835/0.04/150/0, dsn=4.4.1, status=deferred (connect to alt4.gmail-smtp-in.l.google.com[64.233.177.27]:25: Connection timed out)

I figured it was because my ISP was probably blocking port 25, so I changed it in the master.cf file to port 587 to see if it would work, but it didn't. The only time I was actually able to send an email was when I used Gmail SMTP. I would just use the Gmail SMTP server, but it prevents me from using my own domain.

Any help would be much appreciated! Thank you!

Here are the tutorials I followed:

Configure Postfix to Use Gmail SMTP on Ubuntu

How to Install and Configure Postfix as a Send-Only SMTP Server on Ubuntu 16.04

(I don't use a hosting provider, I just host off a machine I own)

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