My Internet Ultimate Classic 300Mbps service runs at 40-150Mbps and no 5G wifi

I cannot get better than between 40-150Mbps service out on my WiFi for the last five months now and my 5G connection constantly drops out.

So I'm paying $120 a month for 15-50% of the connection speed I pay for and can't use my fastest wifi option, instead, I have to use the standard one, which runs at 40-70Mbs.

I've tried calling tech support and restarting the router works for about an hour before I lose my 5G again. Checking the service on all my wifi devices shows the same speeds. I will try adding a $50, 100ft ethernet cable to see if a hard connection is any better than the wifi, but since I know the service was better before, I don't expect that to make a huge difference.

When I initially got it set up, the Cox Speed web tests showed 300Mbps+ on my desktop and two phones, all on wifi connections. Since the pandemic hit, speed has tanked, but for the same price. I can understand if it was maybe 25% slower, but 50-85% is kind of bad, for no adjustment to my bill.

While 40-70Mbps is still faster service than Century Links DSL service, and I can understand people are stuck at home and using more streaming services, reducing bandwidth, all I get from support is "I might have a virus or an older computer or speeds in my area may be reduced during peak times," which is 24-hours a day for five months now?

Even my auto insurer, State Farm reduced my monthly bill by 50% for 3 months since they knew I wasn't driving my car more than 2-3 times a month. Does anyone have any thoughts or suggestions other than me spending more money to hardwire my connection to test a theory?

Thanks