How to make all fonts "Montserrat"

I’m a beginner in the WP world. Activello is the first theme I’m creating a child theme for (at least, trying to). I need to make the fonts that aren’t Montserrat disappear.
So, I looked through a bunch of topics about changing fonts and first, tried to use the “additional CSS” in the Customize section, but there were too many entries and it wasn’t working out. Then, I decided to create a child theme and erased “Lora” and “Maven Pro” from the CSS, making “Montserrat” the font under “body”, “h1, h2. etc”, “p” and “a”. It still didn’t work: Maven Pro was still appearing on the title and Lora, on the sidebar. I tried tweaking a little and reuploading the child theme, but now WP says it’s broken (much like my heart, after all this conundrum).
I don’t know if this is best practice, but since the only font I wanted was Montserrat, I downloaded it and put it in a folder called “fonts”. Since I couldn’t find on the WP Codex how to put them in the functions.php file of the child theme, I came up with a way.
The thing is, the child theme runs perfectly in my local server, but when I try to upload to WP, it doesn’t. First, it says “You need the parent theme”. So, it installs Activello. After that, it says “couldn’t find parent theme” - but Activello is the active theme on the site! I tried uploading again and got:

Downloading install package from…

Unpacking the package…

Installing the theme…

Destination folder already exists. /home/katha623/public_html/rioempauta/wp-content/themes/activello/

Theme install failed.

This is a copy of the child theme’s folder on Google Drive, so you can see the functions.php, style.css and custom.css files for yourselves:
I finally gave up and decided to ask: what would be the best way to do what I need to do?

Thanks for your help,

Hello Katharina,

I hope you are doing well today.

In order to have the child theme, you need to also have the parent theme installed then make the child theme and you activate the child theme instead of the parent theme.

I would recommend that you manually install the theme then use the following plugin to make a child theme, and activate the child theme:

I would recommend that you use the following plugin to change the font to Montserrat because it alloys you to easily manage font and where they are used:

Best Regards,