Thanks for the replies. I found the problem. In a fixed location in the house at 300 Mbps, it has never mattered whether I was using 2.4Ghz or 5Ghz. Both have, and should, do the job. But it turns out that something was wrong with the 2.4 connection. Switching to 5 brought it back to normal performance. The other computers have the same problem using the 2.4 connection, so whatever it is, it's in the router. 
With my ISP I should get about 300 Mbps. The other computers in the house, including another 5502, all get 300 Mbps. I'm getting 30 Mbps. The adapter in both Inspirons is the Intel Wi-Fi 6 AX201. I disabled the internal Intel adapter and installed an external USB Netgear adapter and the speed increased by only 10-20 Mbps.  So, I've ruled out the adapter, driver, ISP, and router. I'm having no other system issues. Both Inspirons use the same A/V software (Bitdefender Total Security) and firewall protection.  So what am I missing? Is there any hardware issue that could affect network speed other than the adapter? This has occurred within the last couple of weeks, and there have been no changes to the system during that time other than my testing for this issue. Thanks for any feedback.
Inspiron 5502 slow Wi-Fi but not the adapter