Les polices sont souvent en OTF. Il se peut que Divi ne puisse les installer, d’où la technique suivante.

1 - Transformer les OTF en tous les autres formats (EOT, TTF, WOFF, WOFF2), grâce à https://onlinefontconverter.com

ou https://convertio.co/fr/otf-ttf (pas de WOFF2 ni de EOT disponibles)
ou https://everythingfonts.com/otf-to-eot (pour EOT)

2 - Ecrire ensuite dans la feuille CSS :

@font-face {
    font-family: 'Core Mellow';
    font-style: normal;
    font-weight: normal; /* font-weight: 300; */
    src: url('/wp-content/themes/Divi-child/fonts/core-mellow/Core-Mellow-55-Medium.otf') format('otf') ,
    	 url('/wp-content/themes/Divi-child/fonts/core-mellow/converted-files/CoreMellow-Medium.eot') format('eot') ,
    	 url('/wp-content/themes/Divi-child/fonts/core-mellow/converted-files/CoreMellow-Medium.woff') format('woff') ,
    	 url('/wp-content/themes/Divi-child/fonts/core-mellow/converted-files/CoreMellow-Medium.woff2') format('woff2') ,
    	 url('/wp-content/themes/Divi-child/fonts/core-mellow/converted-files/CoreMellow-Medium.ttf') format('truetype')
    ; /* woff, woff2 & ttf fctn sous Safari, mais ni otf, ni eot */
}

Pour un ensemble :

/* Light */
@font-face {
    font-family: 'DinNextLTPro';
    font-style: normal;
    font-weight: 300;
    src: url('http://baf.arsenal-productions.com/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-LightCondensed.otf') format('otf') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-LightCondensed.eot') format('eot') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-LightCondensed.woff') format('woff') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-LightCondensed.woff2') format('woff2') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-LightCondensed.ttf') format('truetype')
    ; /* woff, woff2 & ttf fctn sous Safari, mais ni otf, ni eot */
}

/* Normal */
@font-face {
    font-family: 'DinNextLTPro';
    font-style: normal;
    font-weight: 400;
    src: url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-Condensed.otf') format('otf') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-Condensed.eot') format('eot') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-Condensed.woff') format('woff') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-Condensed.woff2') format('woff2') ,
         url('/wp-content/themes/Divi-child/fonts/DinNextLTPro/DINNextLTPro-Condensed.ttf') format('truetype')
    ; /* woff, woff2 & ttf fctn sous Safari, mais ni otf, ni eot */
}

Puis, en donnant le nom de la fonte à la classe du module, par ex. 'didot' pour un module Divi de texte :

.didot p {
	font-family:Didot,Georgia,"Times New Roman",serif;
}

3 - Correspondances font-weight nombre/taille

https://developer.mozilla.org/fr/docs/Web/CSS/font-weight

/* Valeurs avec un mot-clé */
font-weight: normal;
font-weight: bold;

/* Valeurs numériques */
font-weight: 100; /* Thin (Hairline) */
font-weight: 200; /* Extra Light (Ultra Light) */
font-weight: 300; /* Light */
font-weight: 400; /* Normal */
font-weight: 500; /* Medium */
font-weight: 600; /* Semi Bold (Demi Bold) */
font-weight: 700; /* Bold */
font-weight: 800; /* Extra Bold (Ultra Bold) */
font-weight: 900; /* Black (Heavy) */
font-weight: 950; /* Extra Black (Ultra Black) */