IIRC, when I setup the Flirc back in 2019, I had loaded the old Shield's (2017) IR codes into my Pronto, and then used those codes to teach to Flirc. So, trying a new set of IR codes is a possibility, but maybe later down the road if other things don't pan out (e.g. trying RFX9600 firmware 1.4.7 instead of 1.4.8 beta), and trying old Linksys WAP54G.

Other option is to use getHTTP commands from Pronto to the Harmony Hub, which talks to the Shield via bluetooth. This should be reliable, except the most important buttons, the cursor keys, won't repeat properly (when doing press and hold) if they issue IP commands.