Connection between two different subnets

Are both network at the same location?

You might remove FritzBox as gateway and create another interface on Sophos. But… your Setup is unclear.

Can you draw a network diagram explaining your setup?

Is it WAN->FritzBox->Sophos? Or WAN1->Sophos and WAN2->Fritzbox? How are they related to each other?

Yes both networks are at the same location. This is the setup, I hope this is helpful.

Okay, two options:

1. Add route on FritzBox: 10.0.100.0/24 at 192.168.1.55 but might still be a problem because packet takes not same way to and from target.

2. i‘d suggest adding fritzbox as second wan, so you will have high-availability wan using sd-wan as well. Sophos will be gateway for both networks:

The second network (fritzbox) belongs to another company, we could not use this connection for ha wan. I only want to use the printer because the printer is the only ressource that should be shared for both networks. 

you might create s-nat rule, to masq traffic. That would skip fritzbox as gateway for 192.168.1.0/24 Clients.

from 10.0.100.0/24 to 192.168.1.0/24 change source to Port 4 Address 192.168.1.55/32.
Due to this printer will not see 10.0.100.0/24 network. Source will be 192.168.1.55 and send packets back to 192.168.1.55.

Are these the settings you recommend?

No - more like this:

just these three settings - everything else default when creating new rule.

That's it! Thank you very much!