RAX43 (sudden) drop in Wifi Speed

A couple things. 

1. try reinstalling the firmware to it and then doing a factory reset and reinstall. If something was corrupt in either the firmware or the settings, a reinstall and then reset should clear them both out. 

2. I've never seen 2.4ghz hit 300mbps of actual throughput. link speed sure. But actual speed through it, not a single time. 2.4ghz is a much slower network than 5ghz. Plus its more sensitive to interference. If it was link speed you were talking about, try logging into the router and changing what wireless channel you're on. Maybe one of the neighbors routers rebooted in the storm and changed to be on the same channel as yours. 

3. You were hitting 300mbps at over 300ft away from the router?! again, something I've never seen before. My home has the same square footage (roughly) as yours and my whole lot is only 75ft by 225ft (roughly 1/3 acre). If my router is in the home and i'm in the back corner of my lot, my connection isn't fast (tested it). And I'm still not 300ft away. So thats an impressive distance to reach. (not saying your lot isn't bigger or that you don't have a grandmother suite separate from the home, just using mine for example). 

With #2 and #3, I'm a bit more curious whats actually going on because those numbers don't really match up to actual throughput speeds from ANY of the devices I've tested.