Why does my network speed nosedive when using a gigabit switch?

[Combat squirrel]

As title, I need a switch upstairs to route some cables - however iv tried 3 different ones now (granted they are several years old but still a gigabit switch! - a netgear one and TP links) - but when using them the speed drops to 10mb speeds - its connected by a network cable to an Asus RT-AX92U router.

Obviously this isn't a useable speed - however if I connect my PC to this cable, I instantly get maxed out 1000mb (gigabit) network speeds with no issues.

What's going on here? Do I simply need a new / modern switch ?

 

[Combat squirrel]

Thanks all - as WJA says, this is the issue:

Router->long cable->gigabit switch->PC = 10Mbps connection at PC

Router->long cable->PC = 1000Mbps connection at PC

I have connected the cable that was going from pc to gigabit switch DIRECTLY TO the long cable (simply coupled the cables together) and that also works 100% fine at 1gb network, no issue.

Simply adding in any gigabit switch I have into the line ruins it I can't find out why, I guess I could buy a newer switch but its a weird one, my Asus RT-AX92U router is fine as far as I know.

EDIT: Now for some reason its connecting for about 2 mins say, then the connection drops then it reconnects again, all at 1gb but it only seems to stay connected for a couple mins, drops the connection, reconnects again - tried both the Mac mini built in ethernet port and also my call digit ethernet port (tb 3 adaptor), but.....as I write this and I keep an eye on the connection it appears to have stabilised now and isn't disconnecting anymore.....how strange.