You need to have the gallery link to the files themselves; take a look at your status bar when hovering over an image and you’ll see it’s not linking to an image.
WordPress has a bug here, so make sure the shortcode looks like this:
[gallery link="file" columns="5" ids="275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,306,307,308,309,310,311,312,313,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,416,417,418"]
Regards,
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Cosmin
Support Assistant