ERR_CONNECTION_RESET for some users

I have a website on Centos 7.4 running Apache 2.4.6
Recently I'm getting some users unable to view the site. On trying to load any page they instantly get an ERR_CONNECTION_RESET and the page does not load.

I managed to recreate this using my 4G (mobile network) connection on my phone, when I use WIFI on my phone the site works fine.

Any suggestions for things I could check which could be causing this?

I use mod_ssl with virtual hosts if that's relevant.

I cannot find anything in any...

ERR_CONNECTION_RESET for some users

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