If the 60 module is used in SISO mode, what should I do with the unused antenna port?

The unused antenna port must be terminated with a 50 ohm load. A 1/4 Watt resistor is recommended. For the u.fl version of the module, it might be more cost-effective to just use one of the certified antennas. It's not easy to find an affordable 50 ohm resistor that plugs into the u.fl connector.