I'm not a gpg expert by any means but It is my understanding that in general the public key server system has been ruined and it is generally advised to get keys by other means. Without going into trustworthiness of key sources, you can get the public signing key files from the OpenWrt website and --install them rather than doing --receive-keys. Maybe that OpenWrt guide needs to be updated or I am unaware of a keyserver to point to.
It looks like you are trying to install the key for the 21.02 release builds so find that section on that web page and click the download link to fetch the signature .asc file https://git.openwrt.org/?p=keyring.git;a=blob_plain;f=gpg/88CA59E8.asc
install it with gpg --import ~/path/to/file/gpg_88CA59E8.asc
You can check it with gpg --list-keys --fingerprint 88CA59E88F681580
Here is the output of doing same on my computer:
$ gpg --import ~/gpg_88CA59E8.asc
gpg: key 88CA59E88F681580: public key "OpenWrt Build System (PGP key for 21.02 release builds) <firstname.lastname@example.org>" imported
gpg: Total number processed: 1
gpg: imported: 1
$ gpg --list-keys --fingerprint 88CA59E88F681580
pub rsa4096 2021-02-20 [SC] [expires: 2023-02-20]
6672 05E3 79BA F348 863A 5C66 88CA 59E8 8F68 1580
uid [ unknown] OpenWrt Build System (PGP key for 21.02 release builds) <email@example.com>
sub rsa4096 2021-02-20 [E] [expires: 2023-02-20]
EDIT: I also verified the sha256sums file per the guide and it passed.