After this filter is applied I cannot connect to my access point. It fails to obtain IP.
As in, the AP fails to get an IP or what you are trying to connect? I'll assume it's the latter.
I tried to whitelist port 68 and 69 but that that didn't work.
If you intended to allow traffic to a dhcp server, subtract one from both those numbers..
You haven't ...
You try to run Raspbian Stretch on a Raspberry Pi 4. On the release notes you will find:
* Based on Debian Buster
* Support for Raspberry Pi 4 hardware
This means Raspbian Stretch as predecessor from Rasbian Buster does not support Raspberry Pi 4. Before continue troubleshooting please start again with Raspbian Buster.
An even EASIER way of setting up a Raspberry Pi as an AP is to use the pi-ap Github repo which automates configuration of a Pi 3B+/4 into an AP.
Connect your Pi to a free port in your Internet-connected router which has DHCP enabled (which is probably most consumer routers).
Once connected, SSH into Pi on the DHCP assigned IP assigned to eth0 by router ...
The capturing is a job for tcpdump. As you have a bridge, you can capture on either side (eth0 or wlan0), but I would recommend the side where the device you want to monitor is attached.
If you have additional devices, you will want to filter. You can use either the MAC address or the IP address. You can also omit the filter to capture everything.