You will not always get a perfect 512Kbps/512Kbps.
Windows XP (generic registry patches)
https://www.speedguide.net/downloads.php
Windows XP SP2 (if you installed it, here is the fix)
https://www.speedguide.net/read_articles.php?id=1497
Are you sharing this connection with other systems?
Have you tried to connect the modem straight to the system and test?
I would uninstall your USB adapter and reinstall it.
Reboot your wireless router and then power up your wireless system and it should find your router.
Usually if the modem relies on your system its called a WinModem.
Your ISP has to support v.92 to get higher speeds.
Plus, dial-up at 52kbps is still slow,
Since Windows was pre-loaded into your machine than you have a OEM version of Windows and an OEM license meaning that its for one machine only, the original machine. So legally you cannot transfer your license to the new machine.
Illegally you can make it work though,
Port 67 TCP, UDP
Related ports 68, 69
DHCP Server, used to provide clients with network configuration information and other data.
Firewall port 67 inbound and outbound where possible. DHCP traffic should not go beyond subnets, unless a DHCP relay is in use.