VPN assistance required

Hi everyone.

I've a query which I'm sure someone can help me with.

I've just configured my companys' VPN on my iPad and iMac, but can only connect to it when I'm on 3g/4g on either device.

Switch either device to my home wi-fi and it won't connect to the VPN.

Probably something to do with the modem/router settings, but has anyone got any ideas how I need to change settings??

Many thanks for any assistance.