650w is a lot, and looking at the manual for your UPS I think you're exceeding the limits. It even explains what a long beep means. RTFM, as they say.
Rack mounted replacements are fun. I got a used APC which works well. I bought replacement batteries for it, so make sure you budget for that. Mine uses 4 of these https://www.osibatteries.com/vision-cp1290-battery-12v-9ah-sealed-rechargeable