Hello,
In such case, the sequence would be (50000 is new mobile data limit value, measured in MBs):
1. admin01 uci set data_limit.limit.prim_enb_conn=1
2. admin01 uci set data_limit.limit.prim_conn_limit=50000
3. Wait for router's response that both values have been replaced
4. admin01 uci commit
5. Wait couple seconds so that router would have enough time to react to "uci commit" SMS message
6. admin01 reboot