9160/LTE; no return from lte_lc_connect() and keeps resetting

Question

Hi, 
 I'm having trouble with my custom 9160 board. It won't connect to LTE-M network and keeps resetting. 
 My prototype was developed using the 9160DK. I can connect to LTE-M network and send data to nrf Cloud. The combination is 9160 B1A, mfw 1.3.1, and NCS SDK1.7.1. 
 Now my custom board is using B0A silicon, with mfw 1.1.1. I have upgrade/downgrade between 1.1.1 and 1.3.1 multiple times. But that doesn't seem to make any difference. 
 My test code is simplified to like this: 
 
 LOG_INF ( "modem init" ); 
 lte_lc_register_handler ( lte_lc_evt_handler ); 
 err = lte_lc_init (); 
 LOG_DBG ( "lte_lc_init(): %d " , err ); 
 err = lte_lc_connect (); 
 LOG_DBG ( "lte_lc_connect(): %d " , err ); 
 LOG_INF ( "modem init done" ); 
 The log message looks like this: 
 
 00 > I: modem init 
 00 > D: Sending AT command to set system mode: AT%XSYSTEMMODE= 1 , 0 , 1 , 0 
 00 > D: System mode ( 4 ) and preference ( 0 ) configured 
 00 > D: lte_lc_init(): 0 
 00 > D: Sending AT command to set system mode: AT%XSYSTEMMODE= 1 , 0 , 1 , 0

So it went through lte_lc_init(), but never returned from lte_lc_connect(). It will stays there for various amount of time, from a few seconds to a minute or so, then it resets. 
 Since this is a custom board, I try to figure out if any hardware issue could cause such problem. I assume lte_lc_connect() will search for the network and try to make a connection. The document says it has a timeout. I assume if it timed out, it should return from lte_lc_connect() with an error code? But it never returned and always resets after a while. Any possible explaination lte_lc_connect() could cause this? Or should I more look at hardware side? How can I get more debug information to troubleshoot this? I already set CONFIG_LTE_LINK_CONTROL_LOG_LEVEL_DBG=y. 
 The same code runs fine on the 9160DK and get connected in a few seconds. 
 Thanks!