Which?

I infer you mean the wireless one.
Manual
I’m seeing

And mention of Homebase.
So that informs me it is NOT wifi to the doorbell itself, they are using a non-Wifi 2.4Ghz between doorbell and the base, the base then connects to Wifi or Ethernet.
So an extender does not apply between the router and the doorbell, it applies between the router and the homebase.
So given your concerns you could simply place the homebase mid-way between router and doorbell. Or if that does not work, then use a network extension method to get the homebase nearer to the doorbell.
Network extension:
- Wifi extender. Router - extender - base - 2.4Ghz - doorbell.
- old router in bridge mode Router ----- old router … doorbell
- Powerline. Router - Powerline - house electrical cabling - Powerline - base - 2.4Ghz - doorbell.
If you stick to radio signals, so router - Wifi - base, base - 2.4Ghz - doorbell, you can place the base to have as near to line of sight, or non-concrete objects between them.
A good debugging tool is use a Wifi analyzer on your phone and walk around and find a path for the radio to not have to pass through thick walls.