Include Function File in function.php wordpress - php

Question

I just started using wordpress.
I created this function inside function.php, and include file below, works!:
function is_live_2020() {
if(is_page_template('landing.php') || is_page_template('collection.php')) {
return true;
}
return false;
}
include get_template_directory().'/model-filter.php';
but when I created new function again, include file doesn't work:
function is_staging() {
if(is_page_template('landing-staging.php') || is_page_template('collection-staging.php')) {
return true;
}
return false;
}
function load_model_filter() {
if (is_live_2020()) {
include get_template_directory().'/model-filter.php';
} else {
include get_template_directory().'/model-filter-new.php';
}
}
add_action('wp', 'load_model_filter');
how is the correct way to include those function files?
btw.. here model-filter.php:
<?php
function collectionFilter() {
global $wpdb;
$where = array();
if (location_slug()=='my') {
if (1 == $_POST['fman'] && 0 == $_POST['fwoman']) {
$where[] = "filter_Gender like '".(_x_filter_strings($_POST['en'],"men"))."%'";
} else if (0 == $_POST['fman'] && 1 == $_POST['fwoman']) {
$where[] = "filter_Gender like '%".(_x_filter_strings($_POST['en'],"women"))."'";
}
$size = array();
if (1 == $_POST['f28']) {
$size[] = (_x_filter_strings($_POST['en'],"small_size"));
}
if (1 == $_POST['f36']) {
$size[] = (_x_filter_strings($_POST['en'],"mid_size"));
}
if (1 == $_POST['f40']) {
$size[] = (_x_filter_strings($_POST['en'],"large_size"));
}
if (0 < count($size)) {
$where[] = "filter_Size IN ('" . implode("', '", $size) . "')";
etc.....