That depends. In Switzerland, a mortgage agreement doesn't necessarily need to be physically signed. If you already consented to the mortgage in verbal or written form, it could already be considered a legally binding contract. You can find more on that here (in German):
https://www.srf.ch/news/panorama/vereinfachter-abschluss-fuer-eine-hypothek-braucht-es-keine-unterschrift
I imagine this would also be the case if you consent to a mortgage online.
But if the bank specifically sent you a contract for you to sign, then you might be in luck, because the bank wouldn't ask for your signature if it was not required for their mortgage application process.
I recommend you just inform the bank that you decided to go with a different offer. In the worst case they will try to hold you to the agreement, in which case you should present your case to the banking ombudsman. In the best case, they might match or better the lower offer rather than lose you as a customer.