I used this solution and it worked with Font Awesome 5: https://stackoverflow.com/a/50973559/3813846
What made the difference in my case was to add font-weight: 900;to the class. Keep in mind to 'fa' to the value.
Example of my code:
<select class="text-primary fa-select" name="class_logo" required>
<option value="fa address-book"> address-book</option>
<option value="fa adjust"> adjust</option>
<option value="fa air-freshener"> air-freshener</option>
</select>
CSS:
.fa-select {
font-family: 'Lato', 'Font Awesome 5 Free';
font-weight: 900;
}
Edit: If you are mixing Solid Icons with Brand Icons in the select, change the CSS as follows:
.fa-select {
font-family: 'Lato', 'Font Awesome 5 Free', 'Font Awesome 5 Brands';
font-weight: 900;
}