SMSTools3 Help and support

Operating system name and version: Ubuntu Linux 20.04
Version of smsd: 3.1.21
Smsd installed from: package repository (apt in Ubuntu)
Name and model of modem: Huawei e3372 4G LTE USB modem
Interface: USB

I have installed smstools and have followed this article to set it up. But whenever I try to send SMS to a valid phone number, I can conclude that it does not get sent because the messages end up getting stored inside the failed folder, and this is the status of the smsd.log file:

2020-05-31 11:01:33,2, smsd: Smsd v3.1.21 started.
2020-05-31 11:01:33,2, smsd: Running as smsd:dialout (127:20).
2020-05-31 11:01:34,4, smsd: File mode creation mask: 022 (0644, rw-r--r--).
2020-05-31 11:01:34,3, GSM1: Couldn't open serial port /dev/ttyUSB1, error: No such file or directory, waiting 30 sec.
2020-05-31 11:02:04,3, GSM1: Cannot open serial port /dev/ttyUSB1, error: No such file or directory
2020-05-31 11:02:04,2, GSM1: Modem handler 0 terminated. PID: 975, was started 20-05-31 11:01:34, up 0 min.
2020-05-31 11:02:05,2, smsd: Modem handler 0 (GSM1) terminated while mainprocess is still running. Exited, status 127.
2020-05-31 11:39:44,2, smsd: Smsd mainprocess terminated. PID: 974, was started 20-05-31 11:01:34, up 38 min.
2020-05-31 11:45:12,2, smsd: Smsd v3.1.21 started.
2020-05-31 11:45:12,2, smsd: Running as smsd:dialout (127:20).
2020-05-31 11:45:12,4, smsd: File mode creation mask: 022 (0644, rw-r--r--).
2020-05-31 11:45:12,3, GSM1: Couldn't open serial port /dev/ttyUSB1, error: No such file or directory, waiting 30 sec.
2020-05-31 11:45:42,3, GSM1: Cannot open serial port /dev/ttyUSB1, error: No such file or directory
2020-05-31 11:45:42,2, GSM1: Modem handler 0 terminated. PID: 839, was started 20-05-31 11:45:12, up 0 min.
2020-05-31 11:45:42,2, smsd: Modem handler 0 (GSM1) terminated while mainprocess is still running. Exited, status 127.
2020-05-31 14:48:53,2, smsd: Smsd mainprocess terminated. PID: 838, was started 20-05-31 11:45:12, up 3:03.
2020-06-01 11:10:05,2, smsd: Smsd v3.1.21 started.
2020-06-01 11:10:05,2, smsd: Running as smsd:dialout (127:20).
2020-06-01 11:10:05,4, smsd: File mode creation mask: 022 (0644, rw-r--r--).
2020-06-01 11:10:06,3, GSM1: write_to_modem: error 5: Input/output error
2020-06-01 11:10:16,3, GSM1: write_to_modem: error 5: Input/output error
2020-06-01 11:10:26,3, GSM1: write_to_modem: error 5: Input/output error

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Is there anything wrong with that article? What mistake am I doing?

Smstools not working