I want to use SMS as a trigger to enable automation in my home,
That I will send an SMS and it will activate automations in HA
I want to work with this GSM modem integration
Something has information about a good modem that will work with it, and be set from the box to serial mode I do not have the power to mess with settings to turn it into serial mode,
Works great, just pop it into a USB port and it presents itself as a serial device. While I don’t use the official GSM modem integration, it should work just fine. Only caveat with these modems is that they’re 2G only. You need to live in a country / area that still has 2G GSM network coverage. I’m using it to send me SMS from HA using a cheap 2€ / month SIM card.
This could have been a great solution for me
But in my country it has already been announced that support for 2G will soon be discontinued 
Is there such a device that supports 3G the best would be 4G support
Ah that sucks, 4G LTE modems that default to old style serial tty mode will be hard to find, simply because their data throughput rates are orders of magnitude higher than what you’d normally do over an old style serial connection. There are some modules based on the SIM7600X chipset, but they’re pretty expensive and you have to be careful about the frequency bands supported in your country or by your carrier. Some only do Cat 1 IoT bands, which not all carriers or subscription plans support.
TBH, if you need 3G/4G support, your best and cheapest option is to get one of these LTE USB sticks and manually enable serial mode on them (even though this can be tricky).