Second router to extend wifi = slower than a snail, what have I done wrong?!

[#1]

I've just moved house, convinced my wife to ditch our Sky 3Mb 'narrowband' and got Plusnet fibre. 72Mbps. All hail Plusnet! However, my new house has 3 floors and the router has to be situated downstairs where the phone connection is. Net result is very poor wi-fi speeds on the top floor and intermittent wi-fi connection on the middle floor. I have no experience of the Plusnet router so I don't know how good its wi-fi performance is.

So I thought I'd use my old Sky SR102 router to extend the wi-fi range and put it on the middle floor, connected to the Plusnet (PN) router downstairs LAN to LAN. It doesn't have a simple 'bridge' mode so I've had to reset and reconfigure it. I've done everything to the best of my knowledge:

scanned local networks and configured the routers to use channels that don't compete with the neighbours
connected the two routers LAN to LAN
turned off dhcp and IPV4/6 on router 2 and given it an IP address outside of that which would be assigned by the PN router
split 2.4 and 5ghz on the PN router with separate SSIDs and channels (because the auto switching made Apple devices very unhappy)
Used the same SSIDs and passwords on both routers
Configured router 2 to use different channels to the PN router

LAN to LAN speeds with router 2 are great. However wi-fi speeds are terrible. Connected to the PN router on the first floor I get around 40Mbps, which is great, and on the top floor it doesn't work even on 2.4Mhz. If I switch on and connect to router 2 I get 1Mps sitting next to it on the first floor, and obviously nothing anywhere else! I know router 2 used to blast a signal through stone walls 3 feet thick in my old house, so although it is old it was quite capable..

I'm out of ideas as to why the wi-fi performance of the second router would be so abysmal. I know I can buy an extender/repeater but that would create two separate networks and my kids and guests wouldn't know how to switch between the two. I know I can buy a 'mesh' system but they're too expensive at the moment, hence why I thought I'd try my second router.

Has anybody had a similar experience with poor wi-fi speeds on a second router, and did you manage to figure it out and resolve it? I don't know much at all about networking, I've tried to go through every step methodically and rule out various IP/channel conflicts but I may well well have missed the obvious.

Thanks very much for reading this long post!

[#2]

Used the same SSIDs and passwords on both routers

With the same SSIDs I'm not sure how you can determine which router you are actually connected to.

Try using different SSIDs on the SR102

[#3]

Thanks very much for responding, I'll give this a go tomorrow. I also wondered whether I should switch off the wifi on the Plusnet router and just have the SR102 router transmitting on the middle floor. I've no idea if that will have any affect as they're both on different channels but I'll give it a try. Failing that I've read some good comments about the wifi performance of a BT hub 6 which I can pick up off ebay for about £30. It might be a worthwhile last resort.