Q1: How it handles the SIM card switching of UR75-5G? Would it use the backup sim automatically if it had connecting problems?
Answer: Yes, if one SIM drops, it will switch to another SIM card. More information referring article How to Achieve Link Failover in Milesight Router.
Q2: Does UR32 support sending SMS via AT commands?
Answer: No, but you can go to "System->Phone&SMS->SMS" webpage to send SMS or use HTTP API to send SMS.
Q3: Can two SIM card bandwidths of UR75 sum up?
Answer: No, it can't since UR75 can only use one SIM card every time, and another is for backup.
Q4: Whether the UR35 with cellular module admits to turn On/Off relay output by a call?
Answer: No, UR35 DO could not work as a relay output. Only UC11xx/UC3x has relay outputs. Besides, it can be triggered by SMS not call. For more info please refer to article How to Control DO on Milesight Routers.
Q5: Does Milesight UG75 5G support to mmW bands?
Answer: Currently UR75 5G module does not support to it.