What do you do when wifi has a captive portal though requiring a user name and password?

I use my Samsung S10 for exactly this as it has multiple radios that allows connecting to wifi and hotspotting to share that connection with other devices. Great for Chromecasting.

The first device to connect has to go through the captive portal, then the rest doesn’t.

Also works well if your 2nd device doesn’t support captive portals (Chromecast, Tesla car).

I used just clone the MAC address across my iPhone and laptop and switch between them, so I guess you could use your phone to get through the captive portal and then connect with a travel router that clones the phones MAC address.

you would need a router which supports WISP Repeater mode. some GL.inet products support this

I've been thinking about doing somethign similar and am really interested in what hardware you're using for this and what your setup looks like. Do you run a VPN service directly on your router, for example?

I use the TP-Link TL-WR902AC for this, it's cheap and has a MAC cloning feature that makes it pretty easy. Unfortunately the firmware isn't updated anymore and it's missing a VPN client, but apart from that it gets the job done.

Which one do you use?

Not GP, but I've been using the GL.iNet Beryl (GL-MT1300) and I'm really happy with it.

I've been intending to do the same with their Mudi (E750) and between not feeling comfortable trusting their builds and being unable to get my own working (upstream openwrt has stability issues and I guess they have some patches to make the modems work properly; still haven't been able to get a properly working image from working through their repos yet).

It's quite frustrating as it's ideal on paper but they fall on the last mile to make it practically open.