this is the log from the Router
026 2020-01-26 17:39:42 SNTP update time success
027 2020-01-26 17:43:55 system Serial link appears to be disconnected.
028 2020-01-26 17:43:57 pppoe0 pppoe_fsm_down(ppp0)
029 2020-01-26 17:43:58 pppoe0 pppoe_osl_ppp_close(ppp0): close completedly.
030 2020-01-26 17:43:59 pppoe0 pppoe_fsm_up(ppp0)
031 2020-01-26 17:43:59 pppoe0 ppp0 send PADI
032 2020-01-26 17:44:02 pppoe0 ppp0 send PADI
033 2020-01-26 17:44:02 pppoe0 ppp0 receives PADO
034 2020-01-26 17:44:02 pppoe0 Send PADR, Enter PPPOE_SREQ
035 2020-01-26 17:44:02 pppoe0 ppp0 send PADR
036 2020-01-26 17:44:05 pppoe0 ppp0 send PADR
037 2020-01-26 17:44:10 pppoe0 ppp0 send PADR
038 2020-01-26 17:44:19 pppoe0 ppp0 send PADR
039 2020-01-26 17:44:36 pppoe0 ppp0 send PADR
040 2020-01-26 17:45:10 pppoe0 ppp0 send PADI
041 2020-01-26 17:45:10 pppoe0 ppp0 receives PADO
042 2020-01-26 17:45:10 pppoe0 Send PADR, Enter PPPOE_SREQ
043 2020-01-26 17:45:10 pppoe0 ppp0 send PADR
044 2020-01-26 17:45:13 pppoe0 ppp0 send PADR
045 2020-01-26 17:45:18 pppoe0 ppp0 send PADR
046 2020-01-26 17:45:27 pppoe0 ppp0 send PADR
047 2020-01-26 17:45:44 pppoe0 ppp0 send PADR
048 2020-01-26 17:46:17 pppoe0 ppp0 send PADI
049 2020-01-26 17:46:17 pppoe0 ppp0 receives PADO
050 2020-01-26 17:46:17 pppoe0 Send PADR, Enter PPPOE_SREQ
051 2020-01-26 17:46:17 pppoe0 ppp0 send PADR
052 2020-01-26 17:46:20 pppoe0 ppp0 send PADR
053 2020-01-26 17:46:25 pppoe0 ppp0 send PADR
054 2020-01-26 17:46:34 pppoe0 ppp0 send PADR
055 2020-01-26 17:46:51 pppoe0 ppp0 send PADR
056 2020-01-26 17:47:24 pppoe0 ppp0 send PADI
057 2020-01-26 17:47:24 pppoe0 ppp0 receives PADO
058 2020-01-26 17:47:24 pppoe0 Send PADR, Enter PPPOE_SREQ
059 2020-01-26 17:47:24 pppoe0 ppp0 send PADR
060 2020-01-26 17:47:27 pppoe0 ppp0 send PADR
061 2020-01-26 17:47:32 pppoe0 ppp0 send PADR
062 2020-01-26 17:47:41 pppoe0 ppp0 send PADR
063 2020-01-26 17:47:41 pppoe0 ppp0 receive PADS
064 2020-01-26 17:47:41 pppoe0 Enter PPPOE_CONNECTED, session_id = 64869
065 2020-01-26 17:47:41 pppoe0 pppoe_osl_ppp_open(ppp0): open successfully.
066 2020-01-26 17:47:41 system start lcp stage
067 2020-01-26 17:47:41 system local IP address 192.47.0.134
068 2020-01-26 17:47:41 system remote IP address 192.47.0.1
069 2020-01-26 17:47:41 system primary DNS address 1.1.1.1
070 2020-01-26 17:47:41 system secondary DNS address 8.8.8.8
071 2020-01-26 17:48:06 SNTP update time success
072 2020-01-26 17:48:06 SNTP update time success
073 2020-01-26 17:50:44 system Serial link appears to be disconnected.
074 2020-01-26 17:50:44 pppoe0 pppoe_fsm_down(ppp0)
075 2020-01-26 17:50:45 pppoe0 pppoe_osl_ppp_close(ppp0): close completedly.
076 2020-01-26 17:50:46 pppoe0 pppoe_fsm_up(ppp0)
077 2020-01-26 17:50:46 pppoe0 ppp0 send PADI
078 2020-01-26 17:50:46 pppoe0 ppp0 receives PADO
079 2020-01-26 17:50:46 pppoe0 Send PADR, Enter PPPOE_SREQ
080 2020-01-26 17:50:46 pppoe0 ppp0 send PADR
081 2020-01-26 17:50:49 pppoe0 ppp0 send PADR
082 2020-01-26 17:50:54 pppoe0 ppp0 send PADR
083 2020-01-26 17:51:03 pppoe0 ppp0 send PADR
084 2020-01-26 17:51:20 pppoe0 ppp0 send PADR
085 2020-01-26 17:51:54 pppoe0 ppp0 send PADI
086 2020-01-26 17:51:54 pppoe0 ppp0 receives PADO
087 2020-01-26 17:51:54 pppoe0 Send PADR, Enter PPPOE_SREQ
088 2020-01-26 17:51:54 pppoe0 ppp0 send PADR
089 2020-01-26 17:51:54 pppoe0 ppp0 receive PADS
090 2020-01-26 17:51:54 pppoe0 Enter PPPOE_CONNECTED, session_id = 64892
091 2020-01-26 17:51:54 pppoe0 pppoe_osl_ppp_open(ppp0): open successfully.
092 2020-01-26 17:51:54 system start lcp stage
093 2020-01-26 17:51:54 system local IP address 192.47.0.134
094 2020-01-26 17:51:54 system remote IP address 192.47.0.1
095 2020-01-26 17:51:54 system primary DNS address 1.1.1.1
096 2020-01-26 17:51:54 system secondary DNS address 8.8.8.8
097 2020-01-26 17:52:07 SNTP update time success
098 2020-01-26 17:52:07 SNTP update time success
099 2020-01-26 18:00:55 system Serial link appears to be disconnected.
100 2020-01-26 18:00:56 pppoe0 pppoe_fsm_down(ppp0)
101 2020-01-26 18:00:56 pppoe0 pppoe_osl_ppp_close(ppp0): close completedly.
102 2020-01-26 18:00:57 pppoe0 pppoe_fsm_up(ppp0)
103 2020-01-26 18:00:57 pppoe0 ppp0 send PADI
104 2020-01-26 18:00:57 pppoe0 ppp0 receives PADO
105 2020-01-26 18:00:57 pppoe0 Send PADR, Enter PPPOE_SREQ
106 2020-01-26 18:00:57 pppoe0 ppp0 send PADR
107 2020-01-26 18:01:00 pppoe0 ppp0 send PADR
108 2020-01-26 18:01:05 pppoe0 ppp0 send PADR
109 2020-01-26 18:01:05 pppoe0 ppp0 receive PADS
110 2020-01-26 18:01:05 pppoe0 Enter PPPOE_CONNECTED, session_id = 64896
111 2020-01-26 18:01:05 pppoe0 pppoe_osl_ppp_open(ppp0): open successfully.
112 2020-01-26 18:01:05 pppd0 start lcp stage
113 2020-01-26 18:01:06 pppd0 local IP address 192.47.0.134
114 2020-01-26 18:01:06 pppd0 remote IP address 192.47.0.1
115 2020-01-26 18:01:06 pppd0 primary DNS address 1.1.1.1
116 2020-01-26 18:01:06 pppd0 secondary DNS address 8.8.8.8
117 2020-01-26 18:01:28 SNTP update time success
118 2020-01-26 18:01:28 SNTP update time success
Every time the connection dropped this entry appears: system Serial link appears to be disconnected.