IPSec SG135 (9.719-3) vs XGS2100 (SFOS 20.0.0 GA-Build222)

Try a RED Tunnel, then you have an interface and can route over the interface. Later, if you migrate UTM, you could move to route based IPsec. 

https://docs.sophos.com/nsg/sophos-firewall/19.5/Help/en-us/webhelp/onlinehelp/AdministratorHelp/Network/Interfaces/REDInterfaces/HowToArticles/REDCreateSiteToSiteREDTunnel/index.html

https://support.sophos.com/support/s/article/KBA-000003075?language=en_US 

Try a RED Tunnel, then you have an interface and can route over the interface. Later, if you migrate UTM, you could move to route based IPsec. 

https://docs.sophos.com/nsg/sophos-firewall/19.5/Help/en-us/webhelp/onlinehelp/AdministratorHelp/Network/Interfaces/REDInterfaces/HowToArticles/REDCreateSiteToSiteREDTunnel/index.html

https://support.sophos.com/support/s/article/KBA-000003075?language=en_US 

Good morning!

Temporarily we have the connection shared via SSL VPN tunnel, but what we are really interested in is to lift the connection between clients via IPsec tunnel.
Or what is the reason why the peer is not responding.

Thanks for your help.

Hello,

without knowing the configuration details of BOTH ends, we cannot help you.
Could you share screenshots of the edit windows of both ends with us?
You could obfuscate the private info but leave the important config parameters for a review.