UART/Serial Connection Troubleshooting

I'm currently trying to connect via UART to an ER605.
My question may be generic though and apologies if this has been answered elsewhere already.
I understand that UART basically = serial, right? I've got a trusty USB-serial adaptor that I've used many places for years (and have also just tested is working to other devices). It's ending in a DB9 connector and I've only connected pins 2 (tx), 3 (rx) and 5 (gnd). I can also cross pins 2 & 3 and verify I can see my own typing in the console (using putty on my PC). Is this correct so far?
I've soldered a header on to the router following the instructions and I've only connected to the TX, RX and Ground pins (ignored the 3.3v). I have read elsewhere that this is all that is needed. I have set to 115200, 8N1 as specified.
When I power the router on, I can either get nothing at all, or gobbledygook text depending on how I switch TX/RX around. However, I can't get past the gobbledygook. Normally I'd change the baudrate, but I can't find any result from this either.
Is there anything obvious I may be missing, or any suggestions of things I might try to get my connection via UART working?

DB9 would mean wrong voltage?
hope you didn't kill your device, old school RS232 is up to 15v..

Oh, so I need a specific USB-UART adaptor then? Damn. I had thought it was just like a serial. I guess it's got the same sort of settings as serial our use, but different voltage. Is that right?
Any recommended adaptors to use?

yup, that's correct.

I've got https://www.amazon.com/dp/B07WX2DSVB, but it's overkill, there are cheaper ones, esp if you skip 1.8v support.

Got it, thanks. I was using this. Learned something new today! Glad I asked. I'll hunt down the right adaptor tomorrow.

Just to close the loop on this in case anyone else is ever searching a similar thing. I found this at my local electronics supplier today:
RS-232 to TTL UART Converter Module.
This allowed me to keep my existing DB9 serial adaptor and converted it to UART. The only downside is that is needs to be powered separately via microUSB connector. The included cable made it easy for me to connect only the GND, TX & RX to the header I'd previously soldered onto the board. I can now get serial console access to the router. Working.

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