Email services will not transfer a 100 MB file. Most email providers limit the size of an email to 10MB, a few 20MB. To send a larger file than your email provider supports you need to use an iCloud email account and Mail Drop→Add an email attachment in Mail on iCloud.com - Apple Support
And there is no such thing as a 1 GB LTE connection; LTE maxes out at around 100 mbps, and few carriers even go that high with LTE.
Apple does not require Wi-Fi for email, but your cellular carrier may refuse to transfer a large file. But make sure that you have cellular data turned on for whatever mail app you are using. And why did you specifically say an “unsecured” WiFi connection? Does Apple NOT let you use a secured Wi-Fi connection?