Ok, thank you, wasn't sure, since MS recommend to replace classics flows with Power Automate, but will try then with classic. Power automate cloud flows works but it must lookup though another table, check if value returned, can fail, etc.
And it doesn't work 2 ways, as will trigger infinite loop, etc. If I assign to Contact I have it triggered and update SIM record. If I use SIM firm and assign Contact, it will trigger another flow run once SIMs field modified in Contact. Then conditions to cancel, many runs, maintenance to keep all
Thought maybe something possible through relationship settings/connections. In my case SIM used in Shipment, Orders, etc, Sales,
Ideally, use case is once user opens SIM records he can see which Contact it was given, when was sold (Sales), shipped (Shipment) etc. And same from other tables.