Generate random machine ids and send to reMarkable update server to find an ID that is approved for a firmware update.

remarkable_tablet_get_update_machine_id.py

#!/usr/bin/env python3

import time
import uuid
import random
import requests

# This script is tested and working as of November 2020.
# It is only a slightly modified version of leezu's script: https://gist.github.com/leezu/2efb1533d453b70db406b58eb6e3a2af
# leezu gets 99.9% of the credit

# INSTRUCTIONS:
# Run this script on any computer with internet and python3 installed.
# Once a machine id is found, see instructions at the end of this script (instructions will be printed to terminal).

# not sure if the UUID of these ones matter
orig_appid = "98DA7DF2-4E3E-4744-9DE6-EC931886ABAB"
orig_bootid = "fddcd3cd-3bf6-4cee-9a0a-31577b58b6da"

# these two DO matter
orig_machineid = "a9662fb5e8ac48dc848681e6fc5fd158" #example id, run 'cat /etc/machine-id' on tablet for yours
orig_oem = "RM100-739-xxxxx" #find in /home/root/.config/remarkable/xochitl.conf as 'deviceid=RM102-xxx-xxxxx'

# this is the request to send; fill in the desired software version and your current version
# in this example, current version is 2.2.0.48 and desire to update to 2.3.0.16
cur_firmware_ver = "2.3.0.16"
des_firmware_ver = "2.4.0.30"
req = """<?xml version="1.0" encoding="UTF-8"?>
<request protocol="3.0" version="{des_firmware}" requestid="{{{requestid}}}" sessionid="{{{sessionid}}}" updaterversion="0.4.2" installsource="{installsource}" ismachine="1">
    <os version="zg" platform="reMarkable" sp="0.0.4.81_armv7l" arch="armv7l"></os>
    <app appid="{{{appid}}}" version="{cur_firmware}" track="Prod" ap="Prod" bootid="{{{bootid}}}" oem="{oem}" oemversion="2.1.3" alephversion="{cur_firmware}" machineid="{machineid}" lang="en-US" board="" hardware_class="" delta_okay="false" nextversion="0.0.0" brand="" client="" >
        <ping active="1"></ping>
        <updatecheck></updatecheck>
        <event eventtype="3" eventresult="2" previousversion=""></event>
    </app>
</request>"""
url = "https://get-updates.cloud.remarkable.engineering/service/update2"


def get_uuid():
    """Generate a random UUID.
    update_engine/utils.cc just reads this from /proc/sys/kernel/random/uuid
    """
    return str(uuid.uuid4())

def get_oem():
    base = "RM100"
    middle = "739"
    end = str(random.randint(50000, 70000))

    return "-".join([base,middle,end])

while True:
    params = {
        "installsource": "scheduler",  # or "ondemandupdate"
        "cur_firmware": cur_firmware_ver,
        "des_firmware": des_firmware_ver,
        "requestid": get_uuid(),
        "sessionid": get_uuid(),
        "machineid": get_uuid().replace("-", ""),
        "oem": get_oem(),
        "bootid": orig_bootid,
        "appid": orig_appid
    }
    req_fulltext = req.format(**params)
    resp = requests.post(url, req_fulltext)

    if "Bad Request" in resp.text:
        msg = f"Bad request. Request content:\n{req_fulltext}"
        raise requests.RequestException(msg)

    if len(resp.text) != 0:
        if "noupdate" in resp.text:
            print(f"\nNo update ---- OEM: {params['oem']}, machine-id: {params['machineid']} ", end='')
            time.sleep(1)
        else:
            print("Found params to get update: ")
            print(f"{params}\n")
            print(resp.text)
            break
    else:
        print(".", end='', flush=True)
        # there is a limit to how many requests can be sent to their server
        # limit was found to be 3 requests every 20 sec
        # decreasing the delay below will just result in more empty responses
        time.sleep(20)


print(f"\nNow replace your tablet's machine-id with: {params['machineid']}")
print("You can do that by running this command on your tablet:\n")
print(f"\tmv /etc/machine-id /etc/machine-id_orig && echo {params['machineid']} > /etc/machine-id\n")
print("After that, on tablet go to Settings -> Software -> set 'Automatic updates' to 'On' -> press 'Check for updates'")
print("Then you should be able to download the new firmware update!")

That sounds odd. Did you try to reboot your device?
If that doesn't help you might have a DNS problem in your local network?! Maybe try using a different one?
Yes, I rebooted and reset the system, but it didn't work. I also tried the hot spots on mobile phones and computers and other WiFi, and I couldn't. I think my system may be broken.