I use two routers for this, one as just a client router to act as the BTwifi client, with an Ethernet connection to a second router some distance away to provide my access point for local devices. This way the transmitter from one will not swamp the receive amp of the other.
I would suggest starting with a router configured as just a wifi client (not a repeater) and connect by wired Ethernet to a PC and see if you can get it working that way.
The client router does network address translation so it doesn't matter how many devices are connected locally they all come through to the BTwifi access point as one MAC address and one IPv4 address.
I use OpenWRT in the client router which is very configurable. But that is an even steeper learning curve than DD-WRT so you may not want to go down that route,
BTwifi signals from neighbours will be weak, you need to make sure you get at least 20dB signal to noise ratio for the connection to stay up and remain stable. An external directional antenna may be necessary. I use a 19dBi flat plate antenna and only just manage to get around a 22dB SNR. If you are in a flat rather than a detached house this won't be such a problem.
TP-link routers are usually pretty good for wireless sensitivity, I doubt if a different model would improve the situation. I use an old TL-WR1043ND and that works well as a client for weak signals even with the stock short vertical stick antennas.
I don't understand why you would need to disable DHCP on the router. That should only apply for it's local LAN and any wifi access point it provides. It's wifi client connecting the BTwifi access point will be a DHCP client to pick up the CGNAT address (100.x.x.x etc.) that the latter provides.
For speed the service is rate limited to 10Mbps send and receive. In practice somewhat less (e.g. 5 to 10 Mbps) should be achievable provided you are the only BTwifi client on that access point and the wifi signal strength is good and you neighbour has a fast (VDSL or FTTP) connection. Sometimes the CGNAT gateway is heavily loaded and the speed is really poor. Then disconnecting the wifi client and reconnecting gets a different CGNAT client address through a less loaded gateway and the speed improves.
I tried a TP-Link router with DD-WRT configured as a client mode (not client bridge) but I could not get it it to work.
I easily get over 30dB in fact two are at 53 & one is at 31
The disabling of DHCP is advice from TP-Link for WDS, they do not have client mode on most of their firmware except products designed as dedicated wireless access points, I tried one of those but it did not work. Most of their documentation is about client bridge config and they suggest disabling DHCP so it picks it up from the bridged router. This will never work for BTWIFI, but it does if you are extending a router you control because you can tell DCHP relay to point to the host router.
As you say BTWIFI use the 100.x.x.x subnet, configured as a class B subnet. I seem to get a different one everytime I connect, I noticed sometimes the lease if 5 minutes and sometimes 30m but these do not explain the loss of connection which can be seconds after I re-login.
The only common factors I have noticed about the 100.x.x.x network is that it creates are that the default gateway and DHCP server are always at 100.x.0.1 of the subnet used and that the DNS are always 192.168.22.22 and 192.168.22.23 as shown below
IPv4 Address. . . . . . . . . . . : 100.96.55.55
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Lease Obtained. . . . . . . . . . : 01 August 2020 13:16:43
Lease Expires . . . . . . . . . . : 01 August 2020 13:46:47
Default Gateway . . . . . . . . . : 100.96.0.1
DHCP Server . . . . . . . . . . . : 100.96.0.1
DNS Servers . . . . . . . . . . . : 192.168.22.22
but if I try to Trace Router to the DNS servers I get
Tracing route to 192.168.23.23 over a maximum of 30 hops
1 65 ms 22 ms 23 ms 100.96.0.1
2 * * * Request timed out.
then request timed out
I get a different subnet every time I connect if connect to BTwifi on laptop, I can't get WAN on TP-Link firmware, on DD-WRT I can get a static one but no internet connection on the router.
Ironically I do have an old V1 version of an TL-WR1043ND, DD-WRT do not support it but Open-WRT do, I do not know how hard it it but I downloaded the firmware files for this router.
I wonder if you can give some pointers on the configuration you used and any important settings, or better still could you do be backup of your config and PM me a cloud link for the bin file (setting default passwords before you save).
Now BT have rewritten the login page for BTWifi which is even more irritating because it uses a drop down field and BT Broadband is not the default option so it makes logging in even more tedious.
This has become a bit of a mission for me, I can still get it working on my old buffalo but it is SO unreliable despite it working solidly a few years ago.
Any help you can offer would be appreciated.
Edited by thinkbfan (Sun 30-Aug-20 21:44:11)