Ubuntu 18.04 cannot install mono

(I've examined the seemingly similar questions I can find and get to know it's not a duplicate..

Error message:

Err:1 stable-bionic/main amd64 libmono-corlib4.5-cil all 6.12.0.122-0xamarin1+ubuntu1804b1 504 Gateway Timeout [IP: 117.18.232.200 443]
Get:2 stable-bionic/main amd64 libmono-system-servicemodel-routing4.0-cil all 6.12.0.122-0xamarin1+ubuntu1804b1 [41.6 kB]
Fetched 41.6 kB in 16s (2,555 B/s)
E: Failed to fetch 504 Gateway Timeout [IP: 117.18.232.200 443]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

Following operations have been attempted and none of them work.

1. apt-get update
2. follow the mono download instructions for Ubuntu 18.04:
   1. sudo apt install gnupg ca-certificates
   2. sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
   3. `echo "deb stable-bionic main" | sudo tee /etc/apt/sources.list.d/mono-official-stable.list
   4. update index
3. install again with fix-missing.

OS info:

> uname -a
Linux **** 5.4.0-87-generic #98~18.04.1-Ubuntu SMP Wed Sep 22 10:45:04 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

EIDT: I can directly fetch the timeout deb package from the url, but when installing, it said:

dpkg: dependency problems prevent configuration of libmono-system-servicemodel-routing4.0-cil: libmono-system-servicemodel-routing4.0-cil depends on libmono-corlib4.5-cil (>= 4.0.0~alpha1); however: Package libmono-corlib4.5-cil is not installed. libmono-system-servicemodel-routing4.0-cil depends on libmono-system-configuration4.0-cil (>= 4.0.0~alpha1); however: Package libmono-system-configuration4.0-cil is not installed. libmono-system-servicemodel-routing4.0-cil depends on libmono-system-runtime-serialization4.0-cil (>= 4.0.0~alpha1); however: Package libmono-system-runtime-serialization4.0-cil is not installed. libmono-system-servicemodel-routing4.0-cil depends on libmono-system-servicemodel4.0a-cil (>= 3.2.3); however: Package libmono-system-servicemodel4.0a-cil is not installed. libmono-system-servicemodel-routing4.0-cil depends on libmono-system4.0-cil (>= 4.0.0~alpha1); however: Package libmono-system4.0-cil is not installed.
dpkg: error processing package libmono-system-servicemodel-routing4.0-cil (--install): dependency problems - leaving unconfigured
Errors were encountered while processing: libmono-system-servicemodel-routing4.0-cil

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