Why am I unable to make IP reservations for printers?

I want to make 2 DHCP reservations on my home router.

Summary

  • ISP-locked router.
  • No DHCP reservation feature.
  • Printers support static IPs.
  • I prefer centralized management.

Details

Router
  • Altice model GR14IG.
  • Supplied by ISP (Optimum).
  • Network configuration page accessible via 192.168.1.1.
2 Printers
  • HP LaserJet (silly name) M203dw (connected via Ethernet cable)
  • Epson ET-2800 inkjet (connected via Wi-Fi)

Accessing the local config IP redirects to Optimum’s router management page where, unfortunately, there is no option to change DHCP settings.

Each printer allows me to define a static IP address, but this is not ideal for me. Is there a viable workaround or best practice in this scenario?