Connect to a bigger pipe.
Many older routers are 100 MB limited. So you have a fast range extender - that guy is extending range from an old router? Or from a cable company-supplied router that may be older?
Garbage In, Garbage Out.
After I upgraded my internet speed from 15MB to 100MB, I was still getting only 15MB.
Spectrum rep came out and measured 120MB at the modem.
I then discovered a software app for my Linksys router that had settings for the “allowable” speed...and changed from 15MB to 200MB so I could get the speed I was paying for each month.
I now have 120-140MB on a regular basis.
Bring the modem upstairs?
The limit for a 100mb port is 100mb. You have a bottleneck somewhere else. I have a seven year old PC with a built in 100mb NIC connected to a cellular router and it gets over 20mb up/down depending upon how busy the internet is. Late at night is usually the best.
Check cables. Rg6 does gb+
This might help used to be real important on old slower connections...
https://www.ibm.com/support/knowledgecenter/ssw_aix_72/performance/tcp_max_seg_size_tuning.html
Your range extender’s physical Ethernet port says this in the specifications:
Ethernet Port: 1 10/100Mbps RJ45 Port
You don’t have a 1000Mbps port, so your upstairs computer will never self-select for 1000Mbps.
Have you tried turning it off and on again?
This is where I got the information:
https://static.tp-link.com/2020/202005/20200527/RE305(US)%203.0%20Datasheet.pdf
Your speed is only as fast as the slowest connection.
Many modems and routers have 100Mbps external ports and 1Gbps internal ports. So, internal network communications (like between computers or network attached storage - NAS devices) can transfer at up to 1Gbps (assuming both devices have 1Gbps ports. However, downloading through a 100Mbps external port, even with a 1Gbps download internet speed, would only be 100Mbps.
You should be able to use Cat 5e (not Cat 5) to transmit at those speeds, but Cat 6 would be better (up to 10Gbps).
WiFi extenders typically cut the transmission speeds in half. They’re designed more for getting a signal where it doesn’t typically reach, not for maintaining speed.
If you’re able to verify the speeds up to the WiFi extender, you might want to consider replacing the WiFi extender with EoP defices. The EoP (ethernet over power) uses existing AC wiring to transmit network communications. While they have some that supposed to transmit at 1Gbps, the best I have gotten is in the 700Mbps range - which is pretty good considering it’s an option to having to run network cabling where it might not be possible.