Hi @BrianDuncan ,
I think the best play is to consider your requirements and whether you should purchase anything at all. If you already get 20 Mbps (2.5 MB/s), this should satisfy the requirements of most users and should be suitable even for 4K streaming of Netflix and Disney+ (they specify a requirement of 25 Mbps but I see actual usage as quite a bit lower).
If you require high download speed on your mobile, the cheapest and simplest play is probably a Devolo powerline adapter with WiFi access point functionality (roughly GBP 40 for an older model, you want the larger 550 WiFi plus if you want compatibility with the Devolo app) so that your main router can be 5.0 GHz only and the WiFi AP can be split or 2.4 GHz only. (Possibly any WiFi AP will be fine it's just that a powerline one will have minimal footprint and just needs a short cable to router, the powerline functionality would be unused with a single device but I find sending data over electrical cables to work very well if you choose to do this in the future and place the AP in a second location.)
A capable router can be quite expensive and you will lose considerable time in configuring it to achieve something that is more easily done by adding a WiFi access point to meet your requirement as the current WiFi access point/router does not meet your requirement as you imagine it to be.
Realistically, most people should just make do with 20 Mbps on their phone. Good luck!
If your current router has different WiFi "modes" (e.g. Mode 1, Mode 2) these most likely refer to the combinations of protocols supported and by trying each one, you might find a happy medium where the small devices can still connect but the mobile is forced to connect on a newer protocol that might be a little bit faster per the table above.
