wifi speed problem on iphone's and macbook's

I have some more suggestions or explanation attempts, while I cannot confidently say it will help you out any further. there have been virtually no solutions suggested online, mostly just "new equip", basically.

1. You said you've tried setting your extender to new channels manually. I assume that you have read into the matter and such chosen at least 6 and 11 when testing.
2. to really further troubleshoot, if expected, you'd have to temporarily set your extender to its own wifi SSID, to make sure your devices are correctly connected, and not trying to keep the connection to the router
3. I though will reiterate that your TP-Link device appears somewhat outdated, especially not supporting 5GHz nor 802.11 ac/ax, which are already no longer top of the line.
4. Apart from that, let's assume you have installed the latest firmware for your TP-Link device. The latest version seems to be V5.8_1.0.11 Build 221123, according to the TP-Link web page.
5. If that is the case, I will have to agree with you that either, your laptop has a significantly better antenna/WiFi card, or that
6. there are genuine compatibility issues with Apple devices and older protocols, as 802.11 b/g/n.

I have read another community post in which it was said that (relatively) recent Apple devices have had a genuine drop in 2.4 GHz performance as well.

If you really needed a solution for your speeds, I could only recommend you to get yourself a new WiFi-extender with more advanced capabilities, especially 5GHz broadband.

Edit/Comment: what I meant by "other devices connected" was e.g., your dell laptop being connected to your extender while performing the Speedtest on your apple devices. but if the difference is as high as 80mbitps if the max reached is 100, I do not think your laptop will occupy all that transfer speed when testing your apple devices.

Thank you so much for your consideration.

For the start, I will say that 802.11b/g/n equipment may occasionally suffer interference from microwave ovens, cordless telephones, and bluetooth devices, which is not necessarily the problem in this case. While, the extender might not be capable of supplying your Apple devices with the appropriate level of network connectivity, at least with the current settings, as they should all be able to connect to WiFi Networks up to 802.11 ax, so WiFi 6.

You are nonetheless right to assume that 10-20 Mbitps on your devices are quite low, especially as it seems to work perfectly fine with your Dell laptop, and Apple themselves claim that even on 802.11 b/g/n networks, their devices should support up to 144 Mbitps data rate.

If you can ensure that your Apple devices are connected to the extender when running the test (which they should be as signal strength is about 90/100, if I got that right), that there is no other device connected that may interrupt data rates, e.g. your laptop being connected to the network when testing the speed of your phone, etc, that you are running the tests right after another, so we can safely assume the provider's side is not the issue…

You could also turn wifi on/off… (on your apple devices)

forget and reconnect to the network…

check if your devices generally have low priority on the network/extender…

reset your apple device's network settings…

restart your router…

check any more advanced settings…

So… maybe your network availability is just insufficient…

but maybe there's another problem, with any of your devices, and I unfortunately cannot tell.

I do highly suspect the issue is with some external factor not based on your devices, but as I said, I don't know for sure. I've just recently had the case that when connected with one access point (we have a mesh network), I'll have 1 Mbitps download speed, with another I'll have 650, so…

Maybe any other user has a suggestion, still.

I will thank you again for the help, though.

Edit: can you try changing any of those settings in your router…?

Channel -> Auto, for example, or if your router is capable of it, change the Mode to any newer standards which may assist your apple devices?

just also, you could try if setting your channel width higher could improve your performance, but this depends on the surrounding interference, especially through other people's wifi networks.

do you have any more technical details while I'm looking at it?

as such, more details about the WiFi Network, Extender, or Linux Hardware?

also, are you aware if you are connecting to a 5GHz or 2.4 GHz Network with your devices?

et cetera